I've done a little more testing. For me, this error seems to occur whenever I try to open an already-annotated PDF for editing in the iOS app after annotating the PDF in the Windows app beforehand. The error does not occur when I annotate in the iOS app and then reopen it for editing in the iOS app.
So, it's like this:
Annotate PDF in Windows app, reopen annotated PDF in Windows app = pass
Annotate PDF in iOS app, reopen annotated PDF in iOS app = pass
Annotate PDF in iOS app, open annotated PDF in Windows app = pass
Annotate PDF in Windows app, open annotated PDF in iOS app = FAIL
Please fix it! I'm desperate!
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iOS Version 10.1 (1107121)
Windows Version 6.25.1.9091 (309091) Public (CE Build ce-62.6.10954)