A letterhead applied to a plain document. An “APPROVED” stamp on a form. A draft mark across every page of a contract. These look like three different jobs and are one: compositing two documents into the same page area.
Understanding what happens at that composite is what determines whether the result is readable.
How a page gets drawn
A PDF page holds a content stream — an ordered list of drawing operations. Set this font, place these glyphs here, draw this image there, fill this rectangle.
Crucially, the order is the order things are painted. Later operations paint over earlier ones, the way physical ink would.
Overlaying two PDFs combines their drawing into one page. Which document is drawn first decides which one ends up underneath. Overlay PDF takes the first page of the second file, places it over or under every page of the first, centred and at the fit you select, and exports a separate combined copy.
That single choice — over or under — is the whole difference between a usable document and an unreadable one.
Underneath is usually right
Place the overlay under and the original document’s text paints on top of it. A letterhead, a background, a faint watermark: all of them belong underneath, because the text must stay legible and the mark must not compete with it.
This is what people almost always want and rarely think to specify. A logo composited on top of a paragraph obscures the paragraph, no matter how tasteful the logo.
Under has one catch: it only shows if the base page leaves room. A scanned page is usually one picture covering the whole page, so an overlay placed under it disappears completely. For a scanned document, put the overlay over and keep it sparse.
When on top is correct
Placing an overlay above the content is right in a narrower set of cases:
- A stamp in a clear area — an approval mark in an empty margin, a received date in the corner.
- A mark that must not be missed — “VOID” across a cheque, where obscuring the content is the point.
- A genuinely transparent mark, faint enough that text reads through it.
The test is simple: if the stamp lands anywhere text lives, and it is not almost transparent, it will make that text harder to read. Put it underneath or move it.
Overlaying two PDFs with one translucent
You can, as long as the transparency is already in the PDF you lay on top. Filozy’s Overlay PDF has no opacity slider: it draws the page content of the second file as it was made, and keeps any transparency that content already has. Comments, comment stamps and form fields on that page are not copied, so run a stamp made with a comment tool through Flatten PDF first.
Two things decide what shows through:
- Empty areas are already see-through. A PDF page has no paper of its own; only what is drawn on it covers anything. A letterhead that is just a logo and an address line leaves the rest of the page visible, even when laid over the top.
- Filled areas cover. A scanned page, or a page with a filled background, hides everything under it when placed over. Overlay PDF adds no transparency of its own.
So to get a faint mark, make it faint where you design it: set the logo’s or the text’s transparency in the application you use, then export that page to PDF and overlay it. A light tint without transparency is still solid, so it covers text wherever it overlaps; use it only under the document. For a line of text such as DRAFT or CONFIDENTIAL there is nothing to prepare: Watermark PDF draws it at a preset low opacity.
If the reason you wanted one document translucent was to see how two versions differ, see the layout-shift tip under Practical uses, below, and Compare PDF.
How to line up two PDFs
Two PDFs line up exactly when their pages are the same size and you choose Actual size. Overlay PDF centres the overlay on each page, measured against the part of the page a reader sees rather than the full sheet, and it turns the overlay to match any page stored rotated. Same-size pages therefore match edge to edge.
There are no offset or corner controls. A small overlay, such as a logo-sized stamp, always lands in the middle of the page. If the mark belongs in the top corner, put it there on a full-size page in the application you design it in, and overlay that page.
When alignment goes wrong — a letterhead that sits too high, too low, or off to one side — the cause is nearly always a page size mismatch.
A4 is 210 × 297 mm. US Letter is 216 × 279 mm — wider and noticeably shorter. An overlay built for one and applied to the other cannot align at the top and the bottom simultaneously; the geometry does not allow it.
The fixes, in order of preference:
- Produce the overlay at the same page size as the target document. Best outcome, no compromise.
- Build the position into the overlay page. Because the overlay is always centred, a letterhead that must sit at the top has to be a full page with the letterhead already at its top.
- Accept scaling only when the overlay is a full-page background where a few millimetres do not matter. Stretch to page matches every edge and distorts the artwork if the shapes differ; Fit inside page keeps its proportions, so unless the shapes match it leaves a gap on two sides; Cover page fills the page and trims what overhangs.
Only the first page of the second PDF is used, and it goes on every page, so two multi-page documents are not paired page for page. Check a page in the middle of the document, not just the first one. Documents with mixed page sizes — a report with one landscape table — misalign only on the odd page out.
Watermarks are the generated case
A watermark is an overlay whose content is created rather than supplied. Instead of preparing a PDF containing the word “DRAFT”, you ask for the text and the tool draws it.
Watermark PDF stamps text diagonally across every page at a preset low opacity, producing a separate copy. Use it when the mark is text; use a full overlay when you have artwork, a letterhead, or anything with layout.
Inspect the mark against both light and dark page content before sharing. A grey diagonal that reads beautifully over white body text can vanish entirely over a dark chart or a scanned photograph — and the pages where it disappears are often exactly the pages you most wanted marked.
What a watermark does not do
It is worth being blunt, because the assumption is common and costly.
A watermark stamped into a page becomes ordinary page content. It is a drawing instruction among other drawing instructions. Anyone willing to open the file’s objects can find and remove it, and tools that do this automatically exist.
So a watermark reliably tells an honest reader “this is a draft, do not circulate”. It does not prevent a dishonest one from removing the mark. If the requirement is that a document cannot be altered, no stamp will do it. Protect PDF can require a password to open the file, which controls who can read it, not what a reader does with it afterwards.
Use a watermark to communicate status. Do not use it as a control.
Practical uses that work well
- Letterhead on a plain document. Prepare the letterhead as a one-page PDF at the document’s page size, apply underneath.
- A stamp on one page. Overlay PDF stamps every page, which is rarely what a stamp wants. Extract that page, overlay it, and merge it back.
- Spotting a layout shift on one page. Extract the same page from each version and lay the revision over the original at Actual size: unchanged text lands on itself, and anything that moved shows doubled. It works only when the top page has no filled background, and only one page at a time. To find what changed in the wording across a whole document, Compare PDF matches the pages by content and lists the words added and removed.
- Numbered forms. A blank form underneath, variable content on top.
Before you send it
- Read a full page of body text in the result. Not a glance — actually read it.
- Check a page with a dark image on it, where a stamp is most likely to disappear or ruin legibility.
- Confirm the page count is unchanged. If it grew, the files were merged rather than overlaid.
- Look at the last page, where alignment problems accumulate in documents with varying page sizes.
Both files stay on your device throughout — which matters, since an overlay job usually involves a letterhead you would rather not distribute and a document you would rather not upload.