Hello everyone..
I Was facing similar problem about applications not appearing in "open in" menu...
Well, even I tried uninstalling and reinstalling applications a few times. It was still making no difference. But I later observed something..(I may be getting superstitious!! Or maybe I missed something completely)
1) I configured my work email to iOS native email client. So from here onwards at least that "Open In" menu started showing, which in non-native clients just refused to appear.
2) I was trying to open my docs in one signature app. When I tried to open any format other than PDF, like for example word, excel etc... this option did not appear... But when I opened PDF, it started appearing only for those attachments which were PDFs...
So in my case, this signature app appears in the "open in" menu only and only when I open PDF..
I Am going to try converting the attachment into PDF first and then trying to open the PDF into that signature app... Hope it works...
We have to keep trying certain dumb tricks till Apple fixes this problem once and for all.... PHEW!!!!