How merging a PDF without uploading actually works

Merging used to mean sending a contract to a stranger's server. It no longer has to. A JavaScript library such as pdf-lib runs inside the browser tab, reads the files you drop in through the File API, and writes a new PDF in memory. The network tab stays empty because there is nothing to send.

The practical limit is memory, not a server quota. A desktop browser handles a combined 100 MB comfortably. On a phone with 3 GB of RAM, keep the total under 30 MB and close other heavy tabs first.

Page order follows the order you add files. Add the cover first, then the body, then the appendix. After merging, open the result and check the page count in the viewer title bar before you share it.

Scanned PDFs merge at the same speed as text PDFs, but the output is only as small as the scans inside it. Merge first, then compress the images if the result is too large to email.