Merging is the most-used PDF operation there is, and it is also the one people are most often asked to do with documents they should not be uploading anywhere — signed contracts, bank statements, medical letters, passport scans assembled into a single application. This one runs in your browser, so none of that leaves your machine.
The order is the whole job
A merge tool that ignores order is useless, because file pickers return files in whatever sequence the operating system feels like — usually alphabetical, which puts "Appendix" before "Introduction". The list above is the output order, and the arrows move documents up and down until it is right. Adding more files later appends them to the end rather than reshuffling what you already arranged.
Nothing is re-compressed
This matters more than it sounds. Merging here copies the page objects from each source document into a new one — the text stays as text, vectors stay as vectors, and embedded images keep their original bytes. Nothing is rendered to pixels and re-encoded along the way.
The practical consequences: text in the merged file is still selectable and searchable, scanned pages are no blurrier than they were, and printing at full size looks identical to printing the originals. Tools that rasterise a merge — some do — hand you a document that looks fine on screen and is noticeably worse on paper, with text that can no longer be copied.
What does not survive
- Bookmarks and the table of contents from the source documents. Page content is copied; the outline that pointed at it is not.
- Form fields lose their interactivity, since two documents can easily define fields with the same name.
- Digital signatures are invalidated. That is correct behaviour — a signature attests to one exact document, and the merged file is a different one.
- Document-level metadata such as author and title comes from the new file rather than from any source.
Password-protected files
An encrypted PDF cannot be opened without its password and will be refused with a message saying so. Remove the protection in a PDF reader first — most will let you save an unprotected copy once you have entered the password — then merge the copy.
There is no limit on how many documents you can combine or how large they are, beyond your own device memory. Since nothing is uploaded, there is no queue and no per-file cap to work around.