There are a number of file-sharing apps available in the iTunes App Store, and my favorite is FileMagnet.
You need both the iPhone app as well as a desktop companion client to communicate to your iPhone. Both your desktop and your iPhone have to be connected over the same local Wi-Fi network, and you need iTunes as well (which presumably isn't an issue).
Once you install the desktop client, you can push files from your desktop over to the iPhone by simply dragging them from the Finder or Windows Explorer and dropping them into to the FileMagnet window. FileMagnet can automatically manage the connection between the desktop and iPhone for you as well. It includes viewers for many popular file types, such as PDF, HTML, and Microsoft Word, Excel and PowerPoint. You can store as many files as you have room for on your iPhone, and they can be as large as you have space for.