I suspect, and I don't want to put words into other peoples mouths, the DR handles pdf's better as it has a larger screen and is designed around a "work" environment. The iliad is a good compromise for general reading and "light pdf" viewing.
For any pdf of A4 or US equiv, the DR works very well. Yes you can read them on the iliad in landscape, but it is a far more satisfying experience on the DR. Screen size matters for easy of reading with pdfs.
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