Merging PDFs: what actually happens to your files
Combining documents is the most requested PDF operation there is, and also the one most often done by uploading confidential paperwork to a stranger's server. It does not need to work that way. Merging is a structural operation — it rearranges the objects inside a file rather than reinterpreting the content — which makes it a natural fit for something that runs on your own machine.
A merge copies, it does not redraw
A PDF is a collection of objects: page trees, content streams, embedded fonts, images, metadata. Merging means creating a new document and copying those objects across, page by page, with their references rewritten to fit the new file. Nothing is rasterised and nothing is re-encoded, which has a consequence worth knowing: text stays text. The merged file remains searchable and selectable, vector diagrams stay crisp at any zoom, and links inside a page keep working.
The other consequence is size. Because nothing is recompressed, the merged file weighs roughly the sum of its parts. Two 4 MB documents produce an 8 MB one. A tool that claims to merge and shrink at the same time is almost certainly rasterising your pages into images, which destroys the text layer.
Order is the part people get wrong
When you select several files at once, the browser hands them over in whatever order the operating system lists them — usually alphabetical, which puts page10 before page2 and buries the cover letter in the middle. That is why the list here is numbered and reorderable: arrange it, then merge. The first document in the list becomes the opening pages of the result.
If a single document needs its own pages shuffled, rotated or dropped, that is a different job — use the page organizer, which shows every page as a thumbnail. A common workflow is to organize each document first and merge afterwards.
What will not merge
Password-protected files. A document that needs a password to open cannot be read, and this tool does not attempt to work around that. Open it in your PDF reader with the password, save an unprotected copy, and merge the copy. If one file in a batch is protected, the others are still merged and you are told which one was skipped.
Form fields and signatures deserve care. Interactive form fields survive a merge, but if two documents contain fields with the same internal name they can end up sharing a value. Digital signatures, by design, break: a signature certifies one exact file, so producing a new file invalidates it. That is the signature doing its job, not the merge failing.
Why doing it locally matters here
Think about what people actually merge: scanned identity documents for a visa application, medical results for a second opinion, invoices for an accountant, a signed contract with its annexes. This is the most sensitive category of file most people handle, and the standard workflow is to hand it to an anonymous server. Here the whole operation is a JavaScript library working on bytes already in your browser's memory — you can disconnect from the internet after the page loads and it still merges.