Yes, Sony readers and Kindles show the entire PDF page in portrait mode. You can zoom, but that sucks. Imagine trying to view a web page designed for large PC monitors on a smartphone. Not a good experience. Yes you can view a PDF in landscape and that makes the page wider but you have to scroll at least once to see the rest of the page.

Bottomline is that PDFs tend not to reflow text very well. Ebook readers are designed for viewing reflowable text. Therefore ebook readers and PDFs are not a good match.

Do you make your own books? You mentioned that most of your books are text only and in PDF format. If you can convert them in ePub, RTF, HTML they will look great on the 350. If the PDFs were created from scanned images, then there is little you can do to make them readable on a 5" eInk screen.

If your books were created by you, which program did you use. Did you use MS Word to create them and then save them as PDFs? If so save them as RTFs or HTML. You can also use calibre to convert to ePub.

Bottomline is that PDFs will generally look like a POS on a small eInk screen. They even look bad on the larger eInk screens in most cases.