Every developer has been asked by a relative, a colleague or a client: "What's a safe website to merge some PDFs?" The honest answer used to be complicated. Most free online tools upload your files to a server, process them there and send them back. For tax forms, contracts or medical records, that is a real privacy question.

Modern browsers can now do this work locally. This post explains how client-side PDF processing works, what its limits are and what to look for in a tool.

The traditional model: upload, process, download

[Your file] --upload--> [Server] --process--> [Server storage?] --download--> [You]

Enter fullscreen mode

Exit fullscreen mode

The risks are well known:

files may be stored or logged longer than you expect

the server could be compromised

you depend on the vendor's privacy policy, which few people read

large files are slow to upload on poor connections

The client-side model

[Your file] --> [Browser memory] --> [Processed file] --> [Your disk]

Enter fullscreen mode

Exit…