Font size won't change for purchased ebook on PRS-505

I have just purchased a PRS-505, successfully authorized it, bought an e-book on ebooks.com using my Mac, downloaded it to my Reader in .pdf format, and opened it. The problem is that the font stays on the smallest size even when I push the change size button. When I hold the change size button, it will get larger but shifts the text into landscape mode. Before I buy any more e-books, I would like to get this straightened out. Thanks for the help.