I recently shared some free pdf splitters and mergers applications and ranked those on my personal experience. In reply to those, Lauri Lehtinen shared free online services of his own that can not only merge pdf files but also can unlock and protect pdf files too.
1. PDF Merge
Another online merger, a unique optional feature is a Java Applet for those who wish to merge more files faster, Java support is detected automatically.
See other free PDF mergers.
2. PDF Unlock
Unlocks PDFs and removes restrictions set by the “owner password”, and even “user password” protected PDFs, the latter only if the password can be provided though.
3. PDF Protect
Adds (user) password protection to PDF files.