Caere PDA scanned paper images are pretty much the same as XDOC, so refer to XDOC. (Caere has been bought by ScanSoft, but a press release says that "the product lines of both companies will continue to be sold and supported".)
Works, but needs the multipage behavior. PDA was implemented to see if, unlike Xdoc, it reports all the ink, but no PDA data file in my possession reports the baseline (that's why the text undulates across a line) or images. Not aware of widespread use of this format.