The new version of xsane (www.xsane.org) adds some significant features. Version 0.99 allows pdf and multi-page output. I was able to build it by taking the ebuild for xsane-0.98a-r1 and adding the line "$(use_enable pdf) \", though I'm not sure that's the right way to go about it.
xsane needs nothing special for its PDF support, as far as I can tell. I already tried it more than a week ago, but I simply forgot to commit it. Sorry and thanks for waking me up :-)