Yes, you really can store files on your iPhone. You can easily keep many documents as datastreams in Safari bookmarks. (It's not as tough as it sounds.) Just download FilemarkMaker. This little Mac utility converts a file into a datastream and them pops it in a Safari bookmark. If you have your iPhone set in iTunes to sync Safari bookmarks, the file will appear on your iPhone the next sync. Any files that Safari reads will work. This includes HTML, Word, Excel, PDF and image files. Some people have saved files up to half a meg in size. Others report issues even with smaller files. There are also reports that keeping a large number of files slows Safari down, since it appears to keep bookmarks in RAM. You'll just have to try and see what works for you. This is a great way to keep documents so you can view them in airplane mode.
You also have the option of apps like Air Sharing. This can store files in your iPhone's memory. The built in viewer can read most common file formats.
Storing Files on iPhone
