Adobe Acrobat uses object streams (PDF 1.6) when it signs a PDF exported
from LO (PDF 1.4), with this we can verify that signature.

If the PDF had at least one signature in LO, then the doc is not
upgraded from PDF 1.4, so that was working already.

Change-Id: I54b4447ca965a8ba1ffc69bde228ab6f0bda59ee