I recently booked a trip and saved my confirmation page as a PDF. I usually print the PDF and take it with me to so that I have all of the important information at hand. With my new iPhone, I wanted to be able to bring the PDF to the phone and not have to print any papers. After surfing the App Store, FileMagnet (US$5) appears to be the only iPhone app that allows you to synchronize files from your desktop to your iPhone.
FileMagnet requires that you download a small desktop client that pairs with the iPhone app. Also, the current version has some issues with Excel, but they are working on it. The program has been working great so far and I would rank it as a Top 5 application for my iPhone.