qPDF Viewer - a free ad-free Android PDF Reader - also has a great text reflow mode. Users can zoom in and out to increase or decrease the font size. Look for "Reflow Text" under the action toolbar "View" menu.
To return to the last page you visited, you can just tap on the screen to notice a bar in the bottom. If you look closer you can see a blue triangle that marks the last visited page. Click on it and you'll return to that page!
Click image for larger version
Hope this helps.
Sounds like a damaged flash card, or a broken file system.
If it's a Windows PC, insert the flash card, then press WinKey+R (or go to Start -> Run) and type diskmgmt.msc, then click OK. You should be able to see your flash card within the list, and see if it's got any partitions listed.
In this case, I've got a 2GB SDHC card plugged in, which shows up as a ...