The default PDF reader is Adobeviewer.
Check to see if your firmware is 15.1 which has issues that neither the earlier 14.2 (generally rock-solid) or 15.2 (not quite perfect but still better) have.
If the pdf file is DRM-free you may also want to press-and-hold to selection button while in the library and get the option to open it with the alternate pdfviewer which some people prefer. (Me, I have no real preference as I avoid pdfs like the plague whenever possible.
)