In short
This combines two or more PDFs into a single file, in whatever order you set, entirely in your browser. It's built on pdf-lib, a pure-JavaScript PDF library — there's no WebAssembly, no plugin and, most importantly, no server. Each source document is parsed locally and its pages are copied into one new PDF you download. Nothing is uploaded, and you can confirm that in the Network tab.
How the merge works
A PDF is a collection of numbered objects — pages, fonts, images, text streams. Merging means creating a fresh, empty document andcopying every page object from each input into it, in sequence, carrying along the resources each page depends on. pdf-lib does this faithfully: text stays selectable, images keep their resolution, page sizes are preserved as they were. Reorder the files before you export and the pages follow that order in the output. The result is one standard PDF any viewer opens.
Why no-upload wins for merging
The documents people merge are rarely throwaway: an invoice bundle for accounting, scanned ID pages for an application, contract sections plus their signed annexes. Upload-based merge sites copy every one of those files onto servers you can't audit, for a retention period you didn't agree to. In-browser merging removes that exposure rather than promising to manage it: the files are read from your disk, combined in your tab, and no copy exists anywhere else. Open DevTools, watch the Network tab, and see that nothing leaves while the merge runs.
Encrypted inputs: unlock first
If any file you add is encrypted, its pages can't be copied until it's decrypted — so the tool won't quietly produce a broken or partial merge. It detects the encryption and tells you plainly to unlock that file first. Send it through Unlock PDF — which removes owner-password restrictions and decrypts empty- or known-password files locally — then merge the unlocked copy here. Honest behaviour beats a silent failure that leaves you wondering why a page went missing.
If a file won't parse, repair it first
Merge can only copy pages it can actually read. If one of your inputs is structurally damaged — a truncated download, an interrupted save, a clipped attachment — pdf-lib won't be able to parse it, and no honest merge tool can invent the missing pages. When the tool hits a file it can't parse, it stops and points you to repair. Looks damaged?Diagnose & repair it, then bring the repaired copy back here to merge.