There's free software called Calibri eBook Management, available at http://calibre-ebook.com/ that can convert PDF to various eBook formats. There's a version for Windows, Linux, and Mac OSX. I have a Kindle, which supports the MOBI format, so I use Calibri to convert PDFs to MOBI before I copy them onto my Kindle.
After you've downloaded and installed the software, use the Add Books menu item to bring PDF books into your Calibri library, then use the Convert Books menu item to convert them to a format that your eReader supports. Large books can take a few minutes to convert depending on the speed of your computer.
Once converted and copied to your eReader, you'll be able to size the fonts. Happy reading!