If I sign an existing PDF document in LibreOffice Draw, or export PDF from LibreOffice Writer and sign it during export, then Adobe Reader does not like the signature: it claims that the page has been modified. The expectation is that the signature checking should pass without any complaints.
I will attach an example document signed by LibreOffice Writer.
It is also possible to export the same document without signatures, and then use the same certificate in jSignPdf 1.6.4 to sign it. The result does not trigger the "pages modified" alert in Adobe Reader DC, so it is not a problem with the certificate. I will attach a document to demonstrate this, too.
Note: by default, Adobe Reader DC does not trust certificates issued by COMODO. To verify my signature, please, in Adobe Reader DC, go to Edit > Preferences > Signatures > Verification > More... and check the box "Trust ALL root certificates in the Windows certificate store for: [X] Validating signatures".
Created attachment 146257 [details]
Test document signed in LibreOffice Writer on Linux
Created attachment 146258 [details]
Test document signed in jSignPDF 1.6.4 on Linux
Created attachment 146259 [details]
Warning displayed by Adobe Reader DC
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Could you please describe, step-by-step, how did you sign the PDF in LibreOffice, mentioning every button click and keypress (except for passwords, of course)? My assumption would be that you have saved the PDF after signing.
0. Make sure that Firefox is not running (not sure if it is relevant)
1. Open LibreOffice Writer on Arch Linux
2. File > Open, open the odt document. Or just type the text and save it as an odt file.
3. At this stage, the document is not modified. Click File > Export As > Export as PDF...
4. A dialog appears.
5. On the general tab, leave all settings as the defaults. However, I remember that, in the past, i have specifically unticked the "Create a PDF form" box on the General tab, and LibreOffice remembers this. Anyway, the settings on the general tab are: All pages, JPEG compression at 90%, reduce image resolution to 300 dpi, no watermark, no checkboxes in the right column.
5. On all other tabs except Digital signatures, leave all the settings as defaults.
6. On the Digital Signatures tab, select my certificate (that is, click Select, double-click the certificate), type the password, leave all other fields empty.
7. Press the Export button in the bottom right corner of the dialog.
8. Type the desired filename for the PDF file, press Save.
This bug is reproducible on Arch Linux but not under Windows 10.
FWIW xmlsecurity/qa/create-certs/ has a shell script you can run on Linux and use the resulting test certificates on both Windows an Linux if you want to reproduce this.
You can also get free email certificates from COMODO, just as I did.
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