PDF signing feature backported from 4.4

Cherry picked from:
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1fe9ee73a758603ee0e1465931352c41ef8bd999
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bfa01d8a8248f7e44675de5ed1f85a1d17105022
bbb18f679f5a50e9c709520d6c3260d3d9db5aa9
c4cc31b5b55019aabad4045c8174b45e1b27073a
7e3c931786c3cbe83ee170b8b0746d141b520ce6
070c93af73df9aa4eb333265c81060d123b530b9
6e91763769a562b88882a4c2a94b1367c6ed4866

Change-Id: Ib4b67eec0fb7f032a395b5463b481a5680287768