HN user
scribd doesn't have pdf. it has documents in flash. i don't use flash (only flash video outside a browser). [scribd] makes me not click the link. seeing [pdf] might make me click the link. the sooner people stop using scribd for things i find interesting, the happier i am.
hah, and then i remembered hearing this talk at the iwp9 in madrid:
"ConcepOntoFs: A Semantic File System for Inferno", http://lsub.org/iwp9/cready/abheyshahplan9.pdf
this paper also has references to other papers on the topic.
luckily, you can implement the idea on top of existing systems. e.g. fuse or plan9/inferno's 9p/styx fs protocol.
"File indexing and searching for Plan 9", http://lsub.org/ls/export/tags.pdf describes mostly text indexing, but the interface to searching is through a file system (unfortunately not described in much detail at first glance).
only now do i remember that wrote something related once: attfs, http://www.ueber.net/code/r/attrfs. that's for inferno, using the styx/9p protocol. i never polished it, am not currently using it, and the interface the file system provides is not ready for casual file browsing, though that's mostly a detail (from an amount of code point of view). the things stored are strings though, not files. but the illusion that it stores files is easy to build.