To help with your initial answer you can get “better” PDF files with Acrobat pro doing the following (if the PDF is a “text” or “text and images” file):
1 - do what ProfJulie says, ad tags to the file;
2 - take out (crop) all the extra white margins you can in the file - in Acrobat 8 is in document, crop pages.
The unique readers that reflow text PDF files today are the Sony prs 505 and the 700.
For all the others, you can just beneficiate from (2).
The Irex readers have big enough screens to read almost all A4 PDF files without reformatting.