pdfx

pdfx solves a small, annoying problem: bundling several PDFs into one file that still opens normally everywhere. It defines an open format that stores multiple PDFs inside a single, backwards-compatible PDF using an embedded manifest, and ships desktop viewers for Windows, Mac, and Linux to read and build them. Open any pdfx file in a regular PDF reader and it still works; open it in the pdfx viewer and you get the separate documents back. It is MIT-licensed and free.

It is an Electron desktop app, so installing it is a download, not a project. The format itself is documented in a spec, which matters more than it sounds: a bundle-multiple-files feature is only useful if the format outlives the app, and a written spec means someone else could implement a reader if this project stalls. There is no server, no account, nothing phones home.

If you regularly hand off sets of related PDFs (a contract plus its exhibits, a report plus appendices) and want one file instead of a zip, this is a clean, free way to do it. It is not trying to be Acrobat. For merging PDFs into one flat document you already have a dozen free options; pdfx is specifically for keeping them separate-but-together inside one file.

The catch is adoption. The format only round-trips for people who also have a pdfx viewer; everyone else sees a normal single PDF and never knows the extras are there. As a personal organizing tool, fine. As a way to send structured bundles to clients, they need the viewer too, and most will not install it.