I recently found a quite good app named PDF Max & now stick with it. It does provide you dictionary look up function for PDFs, but the feature I like most is iCloud sync in Free Version. The Pro Version has some advanced features, but it is $9.99 though, not a good price for me.
Edit: Wow I'm sorry. It seems like this thread has been existed for quite a long time (since 2010). Hope you guys has found your own solution at the moment.
Last edited by rlowe; 10-22-2012 at 11:20 AM.