I realize it's sort of a kludge, but you could always email the documents to yourself. Once they're in your inbox, you can view them anytime offline. When you're at work, you can use iTunes to synchronize your inbox with Mail.app or Outlook to transfer the documents over.
If you use GMail, you can set up a label for these messages to yourself, and create a filter to detect if the sender is your email address. You can then (if you haven't already) configure your GMail account on your iPhone for IMAP access and you'll be able to access that label as a folder.
edit 07/29/09: Apple just released a new application, iDisk, that works a whole lot better, provided you have a MobileMe account. All you have to do is copy your PDFs (or whatever) to your iDisk on your computer(s) and download them on your iPhone using the new iDisk app. As long as you've downloaded a document, you should be able to view it whether or not you're online.