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If PDFKit in a Rails 3 application cannot generate a PDF, first determine which stage is failing: whether the Rails process can find and launch wkhtmltopdf, whether that executable can convert the same input when run directly, or whether it is hanging or failing to load assets from your application. Installing the PDFKit gem does not install the external executable. Check the binary and its output before changing Rails code.
How PDFKit and wkhtmltopdf fit together
PDFKit is a Ruby wrapper around the separate wkhtmltopdf command-line program. A working gem installation therefore does not prove that the executable is installed, compatible with the host, or visible to the process running Rails. PDFKit’s README says it looks for the binary by running which wkhtmltopdf, and documents setting config.wkhtmltopdf when discovery fails or you need to select a specific executable: PDFKit README.
That distinction gives you a practical diagnostic sequence: verify the executable in the Rails runtime environment, run an equivalent conversion outside Rails, and only then investigate Rails-specific runtime or resource-loading behavior. Do not assume a particular install command or binary path: both depend on the operating system and deployment.
1. Check whether the Rails process can find the executable
Check on the host and as the application user
On the machine or container that actually runs the application, check the command and its location:
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which wkhtmltopdf
wkhtmltopdf --versionRun these checks in the same deployment environment and, as far as practical, under the same user and environment as Rails. A command found in your interactive shell may not be on the application’s PATH. If Rails runs in a container, service manager, job worker, or other separate runtime, checking only your development terminal can give a misleading result.
If which prints no path, or Rails reports that no executable was found, establish whether the executable exists on that target host and whether the Rails process can access it. Do not copy a guessed location such as /usr/local/bin/wkhtmltopdf; confirm the real path on the deployment first.
Set the binary path explicitly
When automatic discovery does not find the right executable, configure PDFKit with its actual path. The PDFKit README documents using PDFKit.configure; a typical initializer form is:
# config/initializers/pdfkit.rb
PDFKit.configure do |config|
config.wkhtmltopdf = "/actual/path/to/wkhtmltopdf"
endReplace the example with the path you verified on the application host. Keep it in an initializer if that suits your app’s configuration. For Rails 3, the README places middleware setup in application.rb; do not move the Rails 3 middleware instruction to environment.rb, which is the README’s Rails 2 note. Consult the README for the middleware setup that matches your application: PDFKit README.
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2. Run the same conversion directly
Next, separate PDFKit configuration from a problem in the executable, input, or runtime. The documented command-line form takes an input URL or file and an output file. Try the same source that Rails is passing to PDFKit:
wkhtmltopdf https://example.com/ output.pdf
# Or use a local HTML file:
wkhtmltopdf /path/to/input.html output.pdfUse a real URL or file relevant to the failing request, and a destination the current user can write. The CLI usage documentation describes its input and output behavior: wkhtmltopdf command-line usage.
Capture the exit status and error output
Record what the command prints, whether it creates a PDF, and whether it exits successfully. In a POSIX-style shell, you can save standard error and print the command’s exit status like this:
wkhtmltopdf https://example.com/ output.pdf 2>wkhtmltopdf.stderr
status=$?
printf 'exit status: %sn' "$status"
cat wkhtmltopdf.stderrRun the command in the directory and under the user that can reproduce the issue, changing the input to match your Rails case. An error or hang outside Rails points first to the binary, input, runtime, or resource access—not to PDFKit’s wrapper configuration. If direct conversion succeeds but the Rails request fails, compare what PDFKit invokes and the application process’s environment before changing unrelated Rails code.
3. If it works in a terminal but not in Rails
A successful interactive conversion narrows the problem; it does not prove Rails has the same access or settings. Compare the successful terminal run with the process handling the failing request:
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- Executable path: Does Rails use the same verified binary? Set
config.wkhtmltopdfexplicitly if discovery differs. - Runtime user and environment: Is the application launched with a different user,
PATH, working directory, or service/container environment? - Input and destination: Can that process reach the same URL or local input and write to the output location used for the PDF?
- Actual invocation: Is Rails converting the same page and options as your successful direct test?
Change one difference at a time and repeat the direct and Rails checks. The project documentation supports explicitly configuring the executable path, but it does not identify one universal permissions or environment adjustment that applies to every deployment.
4. If PDF generation hangs or omits page resources
A command that launches successfully can still hang or produce a PDF without images, stylesheets, or scripts. A frequent configuration to investigate is a page that makes requests back to the Rails app while the app is processing the PDF request. If a single development server process is occupied by PDF generation, it may be unable to serve the assets that the renderer requests, leaving the render waiting on its own application.
Check URLs and server concurrency
- Use absolute asset paths. For raw HTML, provide complete paths or domain-qualified URLs so the renderer can resolve them.
- Check whether the page’s images, CSS, or scripts are fetched from the same Rails server that is handling the PDF request.
- If that request pattern is present, consider running with multiple server workers or embedding the resources rather than requiring the occupied process to serve them.
PDFKit’s documentation discusses multiple workers and embedding resources as ways to address resource requests, and recommends absolute and fully qualified asset paths: PDFKit README. These are diagnostic options, not a guarantee: verify that the missing resources are reachable and that the application server setup actually matches the suspected single-process bottleneck.
5. Account for the legacy version stack
Rails 3 is outside the versions listed as supported in the current PDFKit README, which lists Rails 4.2 and later. That makes a Rails 3 integration a legacy configuration; it does not prove that every existing Rails 3 installation will fail. Record the exact versions and build details before deciding what to change.
The wkhtmltopdf repository was archived on January 2, 2023, and its changelog dates version 0.12.6 to June 11, 2020. Those dates describe project maintenance context, not a diagnosis of a particular host or a guarantee of binary compatibility: wkhtmltopdf releases and archive status and wkhtmltopdf changelog.
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When asking for help or comparing environments, include Rails, Ruby, PDFKit, and wkhtmltopdf versions/build; operating system; the exact direct command; exit status and standard error; and whether the same input succeeds outside Rails. Package availability and binary compatibility vary by host, and the project documentation does not settle a universal package command for every environment.
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Troubleshooting checklist
| Symptom | What to check | Next step |
|---|---|---|
| “No executable found” or shell cannot find the command | Whether wkhtmltopdf exists on the target host and is visible to the Rails process | Verify the actual path and set config.wkhtmltopdf to it. |
| Direct command exits with an error | The exact input, output destination, exit status, and standard error | Resolve the binary/input/runtime evidence from the direct run before editing Rails configuration. |
| Direct command succeeds; Rails fails | Rails runtime user, environment, executable path, input access, and output access | Compare the two execution contexts and configure the verified binary path if needed. |
| PDF hangs or lacks images/styles/scripts | Asset URL completeness and whether the renderer requests resources from a busy single-process server | Use absolute or fully qualified paths; consider multiple workers or embedding resources. |
What to include in a useful bug report
For a legacy Rails 3 failure, a short, reproducible report is more useful than saying only that “the command fails.” Include:
- Rails, Ruby, PDFKit, and wkhtmltopdf versions or build information.
- Operating system and how the Rails process is launched, such as service, worker, or container context.
- The exact direct
wkhtmltopdfcommand, its exit status, standard error, and whether it creates a PDF. - Whether the input is a URL or local file, and which resources are missing if the output is incomplete.
- Whether direct conversion works under the same user and environment as the Rails process.
A historical Rails discussion titled “wkhtmltopdf and PDFkit” mentions Rails 3.0.3, but it is an example of the phrasing developers use—not evidence that one cause explains Rails 3 failures generally: Rails discussion thread.
Frequently Asked Questions
Does installing PDFKit install wkhtmltopdf too?
No. PDFKit wraps the external executable; confirm that wkhtmltopdf is separately available to the Rails runtime.
Is Rails 3 listed as supported by the current PDFKit README?
No. The README lists Rails 4.2 and later. That is legacy maintenance context, not proof that every Rails 3 installation fails.
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