In the meantime you can quickly and easily reflow, resize and crop pdf on your PC beforehand using k2pdfopt app and then transfer such optimized pdf to your old reader that should now turn the pages a lot quicker.
http://www.willus.com/k2pdfopt/
k2pdfopt doesn't reflow the OCR layer behind the scanned image (like many ereaders do) but scanned image itself, so there will be no visible OCR errors in such reflowed pdf, very important for scanned scientific pdfs.
There is usually no need for reflowing of A5 sized pdfs because they can be easily read on 6" ereaders in landscape mode zoomed without margins(i.e. margins cropped).
Also no need for reflowing of two-column or multi-column A4 pdfs because there are multi-column modes in ereaders and k2pdfopt.
https://www.mobileread.com/forums/sho...27&postcount=3
As for best ereader; many android readers there (like Nooks) can be rooted for installation of a lot better pdf apps or jailbroken like Kindles or Kobos for installation of KPV(KOReader) app.
https://www.mobileread.com/forums/sho...12&postcount=3
https://www.mobileread.com/forums/sho...d.php?t=209276