The add-stamp tool applies its stamp only to the page numbers the user selected, looping over options.pages.
The add-watermark tool applies its text to every page of the PDF by looping over pdfDoc.getPages(), with no page-selection step.
Adobe's watermark dialog exposes rotation and opacity settings for the whole mark, since a watermark is meant to sit as a background layer across the page rather than as a movable foreground annotation. Add watermarks to PDFs in Acrobat on desktop | Adobe Acrobat
Add StampPDF Stamp vs Watermark Difference
Select the stamp tool when you need to mark specific pages with a bordered box.
Select the watermark tool when you need text to appear across the entire document.
Choosing Page Coverage for Your PDF
Review the page count of your document before choosing an overlay method.
Confirm the stamp output matches your selected page numbers.
Confirm the watermark output covers every page in the file.
Continue with Select multiple pages on mobile or Customize approved text.
Frequently Asked Questions about PDF Stamp vs Watermark Difference
How do I stamp only specific pages?
Select the specific page numbers in the stamp tool before uploading.
How do I watermark every page?
Upload the PDF to the watermark tool to apply text to all pages.
Where do I find the stamp tool?
Open the add-stamp page to begin the process.
Where do I find the watermark tool?
Search the site's tool list for the watermark option instead.