To see what mds and more importantly its child mdworker is actually doing - use fs_usage to log what files it is opening:
sudo fs_usage -w -f filesys mdworker
Though there is a lot of unintelligable stuff in there, it does tell you when it opens a file to begin reading from it. Copying a PDF into my filesystem shows mdworker opening the file then immediately after lots of activity...
p.s. if you want a little less detail, this will just list the open file points:
sudo fs_usage -w -f filesys mdworker | egrep "open"