For documents I use an app called Dropbox and an app called Plain Text. You have to have a Dropbox account but it's free and totally worth it. 2GB of free space. Which is pretty good. Us you can buy more.
All ya have to do is download the Dropbox plug for your desktop. You can simply drag and drop your files on your lap top to your Dropbox folder and once it's in there you can access these files in the Dropbox app on your iPhone.
The only use of the Plain Text app is simply to edit the text files. I like it. And I also find it very convenient.
On another note you can use your phone as a USB flash drive. You do this with a desktop application called "i-FunBox". It is truly amazing and makes the transfers through USB instead of wifi (which is much better than anything that uses wifi for transfers). There are a few select folders in your phones root file system where you can store all kinds of stuff. I even have entire programs stored in my iPhones file system. However the only way to access these files on your phone is with an app you can only get from Cydia. It's called iFile. After you get the jist of using iFile and iFunbox together it gets pretty self explanatory. Lately I have only been relying on iFunBox to transfer files, programs, docs, and basically anything. You can also edit text and document files with iFile too. iFile is truly at the very top of the list of "most important jailbreak applications". It truly is.