OK, so you probably figured out that viewing pdf/manga ebooks without colour on less than 10" screens doesn't work too well.
So, a LCD-like display that offers colour support and a 10" device like the ASUS Transformer may be just the thing. That's a hardware match!
However, dedicated ebook reader an android tablet is not, so that's where market apps/software like Adobe Reader, Aldiko, Mantano Reader, APV PDF Viewer, VuDroid help fill that void. All of these are free, but some may have pro/paid ($) versions as well.
My favourite app (with an explorer interface to boot) is ezPDF Reader ($)!
ASUS offers some of it's own pdf readers, namely MyLibrary or Polaris Office. However, you've already noticed that its GUI is not ideal. Just use an explorer-like app like ES File Explorer to locate your file and then once selected choose your favourite pdf reader from the list presented. You can even set that favourite app as the default for all future choices.
So, until colour e-ink readers greater than 8" are available, I would abandon the dedicated ebook reader idea, and choose a LCD tablet (hardware) coupled with the right app (software) so as to make the experience acceptable.
p.s. I have a 8" colour ebook LCD reader (REB1200) and 8" grayscale e-ink (iLiad v2) and neither can properly display colour pdf/manga due to their inadequate screen size/resolution/colours! BTW, I initially got involved with PDFRead so as to better assist my ebook reader view pdf files!!!
