Yes, Adobe Portable Document Format (PDF) was invented by Adobe, but I’d hardly call it “proprietary”. The previous link is Adobe’s description of the format and describes how they are attempting to standardize the format even more.

Basically, PDF is a good format for storing documents because you do not have to depend on the host system (the computer you’re plugging your USB drive into) that much to be able to open and print the document. There are portable applications, such as Sumatra PDF Portable that will allow you to open and print a PDF document without even having Adobe Reader installed on the host PC system. PDF is also advantageous because you can secure your documents any way you need to. (Security features vary with the PDF creation method). However, the main advantage of PDF is a lovely thing called backwards compatibility. PDF viewers years from now will almost certainly still open the older-format PDFs.

The only real disadvantage to PDF documents are that they cannot be modified on the fly. (Maybe Adobe Acrobat can do this. I don’t know. I just installed Acrobat yesterday.) Any documents that get modified frequently may be better to store in their native .doc or whatever format instead.