Under the hood
How it works — and where its limits are
Merging sounds trivial — staple the files together — but a PDF is a structured document, not a stack of
loose pages. Each file has its own internal catalogue of fonts, images and page objects, and page numbers in one
file know nothing about the other. This tool uses the pdf-lib library’s copyPages mechanism: every
page of every source document is copied into a fresh document, in the order you arranged, and shared resources
such as fonts and images are copied along with the pages that use them. The result is a normal, self-contained
PDF that any reader can open.
What survives the merge. Everything you can see on the page: text, vector drawings, images,
and page sizes. Mixed page sizes are fine — an A4 report and a landscape slide deck keep their own dimensions in
the merged file, because each copied page carries its own page box. Links inside pages are generally preserved
as page content, and the total page count is simply the sum of the inputs.
What does not survive — stated plainly. pdf-lib’s page copying has two known gaps, and
this page will not pretend otherwise. First, interactive form fields (the AcroForm layer — the fillable blanks
in application forms) are not carried into the merged document; the visible page may still show, but the fields
stop being fillable, and values held only in field data can be lost. If you are merging a filled form, open it
in a PDF reader and print or export it to a flattened PDF first, so the entered values become ordinary page
content. Second, bookmarks and the outline panel are not rebuilt: the merged file will not inherit either
document’s table-of-contents tree. If you need a navigable outline, add one afterwards in a desktop editor.
Why the order step matters. Readers meet the merged file in page order, so a contract whose
signed page lands before the terms, or an invoice bundle whose summary ends up last, creates real confusion. The
list numbers are the final positions — check them before merging, especially when files have similar names. If
you merge in the wrong order, nothing is damaged: your originals are untouched in your device, so simply reorder
and merge again.
Privacy and limits. Merging runs entirely in this tab. There is no upload queue, no server
copy and no record of your file names or sizes kept by this site. The practical ceiling is your device’s
memory: merging many large scanned files at once asks the browser to hold all of them plus the result, so on a
phone, merge in smaller batches. Encrypted inputs must have their passwords removed first.