True, I should have said readability rather than legibility, since the latter is a much broader concept. As @rmschne points out, optotypes like LogMAR’s are designed to standardize legibility for medical testing of visual acuity, not enhance it for lexical processing. (The familiar orthography is more of a convenience than anything else—this article about the PseudoSloan font has a good introduction to the history of optotypes and pseudofonts.)
While the old design wisdom that serif typefaces are more legible has been debunked, they aren’t less legible, either. It seems more relevant for DT3’s users that many—if not most—legal documents, textbooks, newspapers, academic journals, dissertations, novels, etc., are typeset with serifs. Research shows that reading serifs can significantly improve memory and recall, and that a reader’s familiarity with and subjective opinion of a typeface plays a role in ‘immersive’ reading sessions. These observations suggest to me that DT3 users would benefit from the option to clip PDFs in whichever style they prefer and spend most of their time reading.
If accessibility for users with disabilities is the concern, the solution is a flexible, customizable approach that inconspicuously accommodates a user’s individual preferences. I love the DEVON apps and appreciate all the hard work that goes into them—it’s an indispensable piece of software that I couldn’t do without. But my time spent browsing old threads on this forum has taught me that sincere feedback about accessibility aesthetics is often dismissed as being frivolous or in bad taste, rather than welcomed as an opportunity to empower users. (Ever consider that someone might come to this forum to request features that accommodate their disability? 
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