File size isn't a problem. I regularly use an 80MB PDF on my Gen3 (a scanned Latin grammar book, in case you were wondering).
Be warned, though; PDFs which are scanned images (rather than text) will display very poorly on a 6" screen if the original page is significantly larger than that. The CyBook has a "pan and scan" mode which allow you to "zoom in" and "scroll around" the page. I'm not sure how the Sony handles it.
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