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Debug a Rails 3.1 PDFKit failure by tracing the document through four separate steps: Rails renders HTML, PDFKit configures and launches wkhtmltopdf, the converter reads assets and renders the page, and Rails returns the PDF bytes to the browser. Check those boundaries in order. The current PDFKit README lists Rails 4.2, 5.2, 6.0, 6.1, and 7.0, but not Rails 3.1, so it does not assure compatibility for this legacy combination. The checks below are practical diagnostics, not a guarantee that every Rails 3.1 environment can be made to work.
Start by locating the failing step
PDFKit is a Ruby gem that invokes wkhtmltopdf to turn HTML and CSS into a PDF. It uses a WebKit-based renderer; it is not a current Chrome-based browser. A failure can therefore originate in Rails template rendering, PDFKit configuration or process launch, the converter’s access to assets, the converter’s rendering behavior, or the HTTP response that delivers the file.
Work through the pipeline rather than changing several settings at once. First verify the executable Rails actually launches. Then inspect the exact HTML given to PDFKit, make its assets reachable to the converter, and finally check the response headers. If the process hangs, investigate requests back to the Rails server.
Confirm which wkhtmltopdf Rails can run
Run the version check as the same operating-system user and in the same environment that runs Rails:
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wkhtmltopdf --versionIf the command is missing, fails, or reports a different version than expected, fix the installation or executable path before debugging the HTML. A shell opened as your personal account may see a binary that a service account, worker, container, or deployment process cannot see.
PDFKit attempts to locate the executable using which wkhtmltopdf. If that lookup fails or selects the wrong installation, set config.wkhtmltopdf in the PDFKit initializer to the absolute path of the intended binary. The PDFKit README recommends manual installation and says its automated installer was removed; use the installation method appropriate to your operating system and deployment environment.
Record the exact output of wkhtmltopdf --version before continuing. The renderer’s behavior depends on the binary that actually runs, not just the gem declared in the Rails application.
Check the HTML Rails renders before conversion
If text, data, layout structure, or an entire section is missing, first determine whether it exists in the HTML Rails supplied to PDFKit. Rails 3.1’s rendering guide documents render_to_string, which returns rendered content as a string. Save or log that content and inspect it independently of PDFKit.
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- Render the intended template. Confirm the action selects the expected template and that the required instance variables or other data are populated.
- Check the layout choice. Verify that the PDF path is using the layout you expect, or intentionally rendering without one. A converter cannot display markup that Rails never included.
- Inspect the saved HTML. Look for missing text, malformed markup, unresolved asset references, and conditional content that differs between the PDF request and a normal HTML request.
- Compare stages. If the saved HTML is already wrong, fix the Rails rendering path. If it is correct but the PDF is not, continue with asset access and converter behavior.
Keep the sample as a reproducible input. It is much easier to distinguish a Rails rendering bug from a converter limitation when the HTML can be tested outside the full request path.
Why are CSS or images missing from my PDF?
The HTML-to-PDF conversion runs in a separate process. A stylesheet, image, font, or script that works in a browser may not be available to that process: relative paths may resolve differently, a host may not be reachable from the server, or the file may not be readable from the converter’s runtime environment.
- Use an absolute URL with a scheme and host, such as
https://example.com/assets/report.css, when the resource is served over HTTP. - Use a complete filesystem path when the resource should be read from disk and is accessible to the process running
wkhtmltopdf. - For relative references, configure PDFKit’s
root_urland, where needed,protocol. The PDFKit configuration example notes thatroot_urlcan help when the external hostname is unavailable from the server. - Test each referenced URL or file from the converter’s environment. A URL that resolves on your laptop may not resolve from a production host, container, or restricted network.
Check the HTML actually passed to PDFKit: asset helpers and environment-specific host settings may have produced different references from the ones in the page you inspected in a browser. Treat stylesheets, images, and scripts as separate resources and verify each one instead of assuming that a successful page load means all assets were available to the converter.
Why does PDFKit hang in development?
A common deadlock pattern involves the converter requesting assets from the same Rails server that is waiting for the conversion to finish. The original request occupies the only available server process while it waits for wkhtmltopdf; the converter requests a stylesheet or image from that server, but no process is free to serve it. The conversion then waits on a request that cannot proceed.
This diagnosis is especially plausible when a PDF request hangs and the HTML references the application itself for CSS, images, or JavaScript. PDFKit suggests running multiple workers or embedding resources to avoid secondary HTTP requests. As a diagnostic, try a minimal HTML document without application-hosted assets. If that completes, test with assets embedded or make sure the development server has enough workers to handle the converter’s requests while the original request is waiting.
Do not assume every long conversion is a deadlock. An inaccessible asset, slow page, or converter issue can also delay completion. Check whether the converter is waiting on a request back to the same server before changing concurrency settings.
Why does the PDF look fine locally but fail on the server?
Compare the runtime conditions that affect the conversion: executable path and version, operating-system environment, permissions, DNS and network access, and the exact URLs or files referenced by the HTML. The converter on a server does not automatically share your workstation’s installed fonts, local files, or ability to reach a development hostname.
- Run
wkhtmltopdf --versionon the server as the Rails service user. - Inspect the HTML rendered in the server environment, not only a local copy.
- Verify referenced resources from that same environment, including any hostnames, schemes, and filesystem paths.
- Check that the Rails process can execute the binary and that the converter can read any local resources it needs.
- Run a minimal reproduction with the server’s binary and compare its output with the local result.
These checks establish where environments differ; they do not prove that a given version or operating system is inherently incompatible. Preserve the exact failing input and runtime details so the difference can be reproduced.
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Check the PDF response content type
If the generated document is valid but the browser displays garbled text, downloads it unexpectedly, or otherwise handles it as ordinary content, inspect the response headers. The response should use Content-Type: application/pdf. Rails 3.1’s rendering guide says rendered responses default to text/html unless an alternate content type is requested, so set the PDF type explicitly on the response path that returns the generated bytes.
Separate this delivery problem from conversion: confirm that the bytes form a valid PDF, then inspect the HTTP response’s content type and delivery behavior. Changing the header will not fix a PDF that was never rendered correctly.
Isolate wkhtmltopdf-specific rendering defects
When the Rails-generated HTML is correct and assets are accessible, reduce the failing page to the smallest HTML, CSS, and JavaScript example that still reproduces the defect. Run that exact case with the same wkhtmltopdf binary outside the full Rails request path.
- If the minimal case fails outside Rails too, focus on the converter version, its rendering engine, fonts, and asset access.
- If the minimal case works outside Rails, compare the Rails-generated HTML, PDFKit options, process environment, and response handling.
Include the wkhtmltopdf version, operating system and version, and the minimal reproduction when reporting a converter issue; those are details requested by the project’s reporting page. Reproducibility is more useful than a full application that fails only under one unrecorded setup.
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Know the limits of this legacy renderer
The wkhtmltopdf project’s status page says: “Qt 4 (which wkhtmltopdf uses) hasn’t been supported since 2015, the WebKit in it hasn’t been updated since 2012.” That history is a reason to avoid assuming current HTML, CSS, or JavaScript behavior will match a modern browser. It does not prove that a particular feature fails in your installation; test the exact binary with a minimal reproduction.
There are two separate legacy concerns here: the current PDFKit README does not list Rails 3.1 among the Rails versions it lists as supported, and wkhtmltopdf relies on an old WebKit stack. Neither fact alone identifies the cause of a specific error. If you are considering replacing the renderer, compare the Rails integration effort, required HTML/CSS/JavaScript fidelity, engine maintenance and security posture, binary deployment burden, and the work needed to reproduce existing PDFs. The available facts do not establish a best replacement.
Or skip the browser setup
ScreenshotNeo is a website screenshot API, not a replacement PDF renderer or a fix for PDFKit. It can be useful when you need a clean image of a web page for visual diagnosis or documentation without setting up browser automation. One GET request returns a PNG, JPEG, WebP, or PDF. The example below captures a webpage as WebP:
curl -G "https://api.screenshotneo.com/v1/shot" -d access_key=YOUR_API_KEY --data-urlencode url=https://stripe.com -o shot.webpSee the ScreenshotNeo API documentation for request options. ScreenshotNeo accepts cookie or consent banners and removes more than 60 known consent platforms, newsletter popups, and chat widgets before capture; each step can be turned off. Bot checks or CAPTCHAs, blank pages, timeouts, failed loads, and cache hits are not billed, and responses identify page verdict and billing status in headers. An MCP server provides take_screenshot, get_page_info, and capture_pdf tools for AI agents and MCP clients. The free plan includes 1,000 screenshots a month with no card; paid plans start at $5 for 3,000 shots.
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Fast troubleshooting checklist
- Command not found or wrong binary: check the version and execution environment; configure PDFKit with the intended absolute executable path.
- Missing text or layout: inspect Rails’ rendered HTML, template selection, and layout behavior before changing converter settings.
- Missing CSS or images: use complete URLs or file paths, configure
root_urlorprotocolfor relative paths, and verify access from the converter’s environment. - Conversion hangs when assets point to the app: test for a single-process development-server deadlock; use multiple workers or embed resources.
- Valid PDF handled incorrectly by the browser: verify the response sends
Content-Type: application/pdf. - Only the server fails: compare its binary, user permissions, rendered HTML, asset reachability, and operating-system environment with the local setup.
- Unexplained rendering difference: test a minimal case outside Rails and record the binary version, OS/version, and reproduction.
Frequently Asked Questions
Does the current PDFKit README support Rails 3.1?
It lists Rails 4.2, 5.2, 6.0, 6.1, and 7.0, but not Rails 3.1; that list does not provide compatibility assurance for Rails 3.1.
Is wkhtmltopdf based on Chrome?
No. PDFKit documents that it uses WebKit, and wkhtmltopdf’s status page identifies its Qt 4 and WebKit history.
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