You can already view PDF files, that is native. Do you want somewhere to store them?
It may be native, but there are several important features missing!
For example, if my PDF file contains a table of contents page with links to other pages/chapters within the same PDF file, they will work just fine in the OS-X version of Preview, or in any version of Adobe Reader.
Those links will not work at all with the internal PDF renderer included on the iPhone.
In both Preview and Adobe Reader, you can specify your desired view (single page, multiple pages, single page continuous, multiple pages continuous...).
The internal PDF renderer on the iPhone only has one such option: single page continuous.
In both Preview and Adobe Reader, there is the ability to go to the next page, or the previous page, and maintain page alignment when in single page view.
The internal PDF renderer on the iPhone only allows scrolling for navigation, and will not "snap" the next page into alignment.
... and, as you already mentioned, having somewhere to
store personal PDF files is a prerequisite!
- M.