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To watermark an existing PDF in Ruby, use HexaPDF: create a one-page PDF containing your logo, then apply that page to the source document as a background or stamp. Use Prawn instead when you are generating the PDF yourself; use CombinePDF when you already have a watermark page and want a pure-Ruby overlay workflow. The right choice depends on whether the PDF already exists and whether the logo should appear behind or above its content.
Choose the right Ruby approach
These libraries address different stages of PDF work rather than offering interchangeable ways to edit any document.
| Library | Best fit | Watermark workflow | Important consideration |
|---|---|---|---|
| HexaPDF | Applying a watermark to an existing PDF | Create a watermark PDF page, then use its CLI to apply that page as a background or stamp. | Its project documentation describes reading and modifying existing PDFs. Check the license terms for your distribution model. |
| Prawn | Adding a logo while generating a new PDF | Place a JPG or PNG on the page with the image method. | Prawn is focused on generating PDF content; it is not the recommended first choice for editing an existing document. |
| CombinePDF | Overlaying a watermark page already prepared as a PDF | Import the logo page and append it to each target page. | The documented page operator places the imported page over the target page; inspect the output for your documents. |
If you have an existing PDF and a logo image, the HexaPDF route below is the most direct recommendation. If the logo is already a PDF page and you want a Ruby-level loop, the CombinePDF route may fit better.
Prepare the logo and choose its placement
Use a transparent PNG when the logo needs transparent areas. A JPG is suitable when transparency is not needed. Before processing a document batch, decide whether the watermark belongs behind page content or on top of it: background mode puts it underneath, while stamp mode puts it above.
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- Background: use when the watermark should sit behind the existing page content.
- Stamp: use when the watermark should be drawn over the existing content.
- Size and position: set the image’s dimensions and coordinates when creating the one-page watermark PDF. Check the resulting page rather than assuming a logo’s source-image dimensions will produce the intended printed size.
Make a copy of the source PDF and work on that copy first. Rotation, transparency, annotations, forms, encryption, and incremental-update behavior can affect the result; the library documentation does not establish universal preservation of every PDF feature.
Watermark an existing PDF with HexaPDF
The workflow has two parts: use HexaPDF to make a single-page PDF from the logo, then apply that PDF to the source file. The image API supports positioning and width- or height-based sizing, and its documentation demonstrates reuse of an image resource on a canvas.
1. Create a watermark PDF from the image
Install the gem in the Ruby environment used for the task, then create a script such as make_watermark.rb. The example places a logo on a new PDF page and specifies its width; adjust the coordinates and width for your page dimensions and logo.
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- Edit text and images without jumping to another app.
- E-sign documents or request e-signatures on any device. Recipients don’t need to log in to e-sign.
- Convert PDFs to editable Microsoft Word, Excel, or PowerPoint documents.
- Share PDFs for collaboration. Commenting features make it easy for reviewers to comment, mark up, and annotate.
require "hexapdf"
doc = HexaPDF::Document.new
page = doc.pages.add
image = doc.images.add("logo.png")
page.canvas.image(image, at: [100, 500], width: 140)
doc.write("watermark.pdf")Run it with ruby make_watermark.rb. The resulting watermark.pdf is the page that HexaPDF’s command uses as the watermark. The coordinate pair and width are examples, not a universal placement: inspect the page and adjust them to suit your document.
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HexaPDF’s CLI command is:
hexapdf watermark -w watermark.pdf input.pdf output.pdfUse stamp mode to place the watermark above the existing page content, or background mode to put it below. The command supports applying one or more pages from a PDF and repeating watermark pages across the input document; the exact option for setting the mode and selecting pages depends on the CLI version, so consult the installed version’s command help before adding flags to an automated script. Keep the input and output paths distinct so the original remains available for comparison.
3. Check the output
Open representative pages from the generated PDF, including pages with different orientations or rotations and pages containing forms or annotations. Check that the watermark is visible at the intended size and layer, that content remains legible, and that document features you rely on still work. Do not assume a successful command alone proves every feature was preserved.
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Generate a new PDF with Prawn
When your Ruby program is creating the PDF, Prawn can place an image directly on its page. Its image API documents JPG and PNG support. This example generates a new PDF rather than editing an existing one:
require "prawn"
Prawn::Document.generate("report.pdf") do |pdf|
pdf.text "Example report"
pdf.image "logo.png", at: [100, 500], width: 140
endRun the script in an environment with the Prawn gem installed and the image file at the given path. Change the coordinates and width to suit the page. If the existing PDF must be retained and watermarked, use the HexaPDF workflow instead of treating this generation example as an editing method.
Overlay a prepared watermark page with CombinePDF
When watermark.pdf already contains the logo page, CombinePDF documents importing that page and appending it to each page in the target PDF. The documented pattern is:
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require "combine_pdf"
pdf = CombinePDF.load("input.pdf")
company_logo = CombinePDF.load("watermark.pdf").pages[0]
pdf.pages.each do |page|
page << company_logo
end
pdf.save("output.pdf")This overlays the imported page on each page in the document. Prepare the watermark page at an appropriate size and position before applying it; inspect the saved file to confirm the visual result. As with the other routes, test annotations, rotations, forms, encryption, and other document features that matter to your workflow.
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ScreenshotNeo is a website screenshot API, not a Ruby PDF-watermarking library, so it does not replace the PDF workflows above. It may help if the asset you need is a clean screenshot of a web page before you place that image into a PDF. One GET request returns a screenshot or PDF; for an image response, for example:
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. Cookie banners, newsletter popups, and chat widgets are removed before capture; bot checks, blank pages, failed loads, timeouts, and cache hits are not billed. Its MCP server lets AI agents take screenshots. The Free plan includes 1,000 shots per month with no card, and paid plans start at $5 for 3,000 shots. Learn about ScreenshotNeo or sign up free for 1,000 screenshots a month with no card.
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- The logo is missing: check the image path and that the script can read the file. Confirm that the watermark PDF was created and contains the expected page before applying it.
- The logo is in front of text when it should be behind: choose background mode rather than stamp mode. If it should cover existing content, use stamp mode.
- The logo is too large, too small, or misplaced: change the image width or its coordinates when creating the watermark PDF, regenerate that PDF, and apply it again. Inspect the output at the size at which it will be used.
- The logo has an opaque box: use a PNG that actually contains transparency. JPG does not provide transparent areas.
- Pages do not all look alike: check page rotation and dimensions, then review representative pages individually. The documented commands do not establish automatic handling that will make every differently oriented page look identical.
- A protected file fails or behaves unexpectedly: determine whether it is encrypted and whether your workflow can process it. The cited documentation does not promise universal support or preservation for encrypted PDFs.
- Annotations, forms, or other content changes: test a copy and verify the document features your users need. Do not treat successful image placement as evidence that all interactive or structural PDF content survived unchanged.
- The CLI command is unavailable: confirm HexaPDF is installed in the same Ruby environment and that its executable is available to your shell. Check the installed command’s help for syntax and supported flags.
Performance, reliability, and licensing considerations
The available documentation establishes the workflow and APIs, but does not give a stable throughput benchmark, file-size estimate, or compatibility percentage. Measure runtime and output size using representative PDFs in your target Ruby environment before setting batch expectations. For reliability, keep the original file, write to a separate output, and validate a sample of pages after each change to the logo, options, or library environment.
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HexaPDF’s project documentation describes AGPL and commercial licensing options. If you distribute proprietary software that includes it, review the current terms and determine which license applies to your use before deployment. Gem APIs and versions can change, so confirm the command help and API behavior for the version installed in your application.
Which method should you use?
- Existing PDF plus image logo: create a one-page watermark PDF with HexaPDF, then apply it as a background or stamp with the CLI.
- New PDF generated in Ruby: place the JPG or PNG with Prawn’s image method while generating the page.
- Existing PDF plus prepared PDF watermark page: import the page with CombinePDF and append it to each target page.
Frequently Asked Questions
Can Prawn add a watermark to an existing PDF?
Prawn is focused on generating PDF content. For an existing document, the recommended route here is HexaPDF; Prawn is appropriate when you are generating the PDF yourself.
Can I place the watermark behind the PDF content?
Yes. HexaPDF’s watermark command supports background mode for placement below content and stamp mode for placement above it.
Does this guarantee that annotations, forms, and encryption are preserved?
No universal preservation guarantee is established. Test copies of representative documents and verify the features your workflow depends on.
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