Hi Rotha, welcome.
Your issue isn't with the 511EB really, it's with converting from PDF to ePub, which is generally a hit or miss exercise as PDF format documents are laid out with the printed page in mind and the text isn't held in a way that makes it easy to pull out and convert.
I assume you've tried the Reflow setting on your Elonex? (Open the PDF, then Menu > Reflow). If that works reasonably well you wouldn't need to do the conversion.
Can I just check as well that you installed the firmware upgrade
How complex are your PDF documents (e.g. single column, multi-column, etc)?
Can you get those documents in a format other than PDF for your conversion source?
If you want to read PDFs formatted for A4 or Letter on an e-reading device without mucking about with zoom or squinting, you really need a 10" screen or bigger.