As you've found, XPDF is only suitable for non-DRM'd Adobe PDF books.
We chose to include the option of reading PDFs with XPDF on our ECO Reader firmware, as XPDF processes and displays PDFs differently to Adobe.
XPDF zooms the page, while maintaining the aspect ratio and formatting of the page. Adobe zooms by resizing the font and reflowing the text.
If ECO Reader owners don't want the option for using XPDF, we can remove it with a firmware update.