So, you've got an iPhone and all these gigabytes of space but yet there is no native way of using it as a hard drive.
DataViz has been teasing iPhone users about porting its Documents To Go application to view and edit MS Excel, Word, Powerpoint and PDF files. But so far, nothing.
Right now, some of your options in the App Store are:

Right now on the App Store, the maker of
Memory Stick is offering its app for free for a limited time.
It is very similar to Datacase and Air Sharing. Although the description is a bit misleading, you NEED a Wi-Fi connection to upload files. If you don't have either Datacase, Air Sharing or another paid file sharing app I would suggest picking up Memory Stick for free.
Here are my thoughts on the apps I mentioned above:
Datacase: Bookmarking feature is highly useful. With a PDF that has 100 pages or more, bookmarking is almost necessary because scrolling through PDFs on any app seems to be more sluggish than any other file type. Page Up/Down and First/Last Page buttons are also useful.
Air Sharing: Has a slider to move faster through a document as well as Page Up/Down buttons. Makes use of the double-tap zoom in. Slightly better scrolling performance than Datacase.
Box.net: Best solution for someone who does not have Wi-Fi capability. No landscape mode. Storage capacity is 1 GB if you don't want to pay for more. There is also an upload restriction.
Memory Stick: Utilizes double tap for zooming in. Is currently a basic file sharing app.