DRM is encryption added by publishers. If any of your PDFs can only be read by Adobe Digital Editions, then they have Adobe DRM. This is ok because the Opus and the EZ Readers support Adobe DRM (for both PDF and ePub formats). Note that the BeBook is essentially identical to the EZ Reader (both are Hanlins, and both have 6" and 5" devices).
To test if your PDFs need to be shifted to a different ebook format, I suggest trying out Sony eBook Library
on your Mac (I have only used the Windows version, but I assume the Mac version is similar). Its viewer works for PDFs almost exactly like mobile Adobe Digital Editions on EInk devices. If the PDFs look ok when you select a non-default font size (M, L, XL, XXL) in eBL, then they will work ok on an EZ Reader (say). I know you don't want a Sony, but you are only using eBL for testing. Also, EInk devices typicaly have a landscape mode, which is missing from eBL.
Note that there might be a "best" PDF viewer in your list, but in general there is no "good enough" PDF viewer in the current crop of 5-6" EInk devices. They can all be pretty bad on a significant fraction of all PDFs. However, PDFs created as ebooks are usually better that those created from scientific journals (for example).