Thank you both (Borisb and Cheyennedonna) for the very helpful information. I am easily able to access library .pdfs with Repligo. Works like a charm. But, although I can "see" the ext sd card files with ES File Explorer, Repligo says that the .pdfs are damaged and cant be opened. Same thing if I try a .pdf from a USB memory stick. Another .pdf reader gave the same results. Accessing the library files through ES File Explorer and opening with Repligo works great, except as noted below.
Another interesting observation, that may help some people. I have lots of scanned .pdfs and for the "big ones", I use a feature in Adobe Acrobat 9 Professional that allows you to reduce the file size, presumably by stripping away alot off "unnecessary" stuff, by specifying a minimum version number for compatibility. By setting the version number high, such as to Acrobat 9, I can often reduce an 18MB file down to 3MB. However, once the reduction process is done, Repligo is completely unable to read the file (every page is empty with a big Red "X"), although the Library reader has no problems.