There is no document viewer for iPhone installed on it. Although, your iPhone can read Microsoft Word documents and PDF documents if they are attached to your emails. Since the iPhone already has the capability to read .doc and .pdf documents it shouldn’t be to hard to write an application, a document viewer for iPhone that would let you view documents stored on your iPhone.
There is also a problem if you want to save the documents you received by email to your iPhone. You need an email service that pushes the email to your iPhone and not one that just lets you view your emails and leaves them on the server. For this you can try Yahoo.
Other trick to store and view .pdf documents on your iPhone is to print your .pdf as a picure and view them in your iPhone Photo application. You have to save PDF to iPhoto in your Preview app on OS X, type in the name of the document, save it to the folder that is also selected in iTunes and then sync your iPhone. When you browse photos on your iPhone you will also see the PDF broken down by pages. You can view, scroll, and zoom it like any other photo. This is a nice document viewer for iPhone workaround.