Merge PDF files

Merging PDFs properly, and the things that go wrong

Merging sounds like the simplest operation in this whole toolbox, and mostly it is: every page of every file is copied into a new document, in the order shown in the list. The parts worth understanding are the ones that are not obvious — what happens to the things a PDF carries besides pages, why one file occasionally refuses, and why doing this in your browser rather than on somebody's server matters more here than almost anywhere else on this site.

Order is the whole interface

The list order is the output order — the arrows move a file up or down, and that is the only thing deciding how the finished document reads. Two habits save time. First, name your files so they sort correctly before you add them: a scanner producing scan1, scan2 … scan10 will sort scan10 directly after scan1 in most file pickers, which is the classic cause of a merged document with its pages inexplicably shuffled. Zero-padding the numbers fixes it permanently. Second, check the first and last page of the result rather than the middle — an ordering mistake almost always shows up at a boundary between two files.

Why a password-protected PDF stops the merge

An encrypted PDF cannot be read without its password, and that is the point of encryption rather than a gap in the tool. If a file is protected, the merge stops and names it, so you know exactly which one to deal with. Open it in a PDF reader with its password, save an unprotected copy, and merge that. Note the distinction that catches people out: a PDF can be encrypted so that it needs a password merely to open, or so that it opens freely but is restricted against editing. Both are encryption, and both will stop a merge, even though the second kind opens without you ever being asked for anything.

What survives the merge and what quietly does not

Page content survives exactly — text stays text, images keep their original quality, and nothing is re-compressed or re-rendered, so a merged page is byte-for-byte as sharp as it was. What does not necessarily carry over is the machinery around the pages: bookmarks and the internal outline of the source documents, some interactive form fields, and any digital signature. A signature in particular is invalidated by design — the signature attests to one specific document, so the moment that document becomes part of a larger one the guarantee no longer applies. That is not a defect, it is exactly what signing is for. Keep the signed original alongside the merged copy whenever it is a legal document.

File size, and why the result is not always smaller than the sum

The merged file is roughly the sum of its inputs, because nothing is thrown away. It can be marginally smaller when the same font is embedded in several source documents and is stored once in the result, but do not count on a meaningful reduction. If your merged file is too large for an email or an upload limit, the cause is nearly always scanned pages stored as full-resolution photographs — a twenty-page scan can outweigh a five-hundred-page text document. Compressing the images before merging, or scanning at a sensible resolution in the first place, does far more than anything the merge itself can do. Text-based PDFs are already small and there is little left to win there.

Why this runs in your browser

Everything happens on your own machine: the files are read, combined and written to a new PDF without any of it leaving the tab. That is a deliberate choice, and merging is the tool where it matters most, because of what people actually merge. Invoices, contracts, medical results, bank statements, tender documents, signed agreements — the everyday reasons to combine PDFs are almost all documents you would not hand to a stranger. Uploading them to an unknown server to save yourself a click is a poor trade. It also means there is no queue, no file size limit imposed by us, and no waiting: the only constraint is your device's own memory, which on a modern machine is generous.

How merging works here

Every page of every file is copied, in the order shown in the list — use the arrows to reorder files before merging. The pages themselves are untouched: no recompression, no quality loss.