After signing the document and attempting to save, the following error appears:
"Operation failed because Adobe Acrobat encountered an unknown error."
Sorry for taking so long in responding to your query.
Based on the issue description mentioned above, when trying to save the pdf after signing, it shows "operation failed because Adobe Acrobat encountered an unknown error, is that correct?
Please check if an update is pending for Acrobat. Click "Check for Updates" under Help.
You may also download updates manually from these links:
If that doesn't work, try printing pdf to Adobe PDF. Open pdf file in Acrobat> click File > Print and select Adobe PDF from the printer's list.
Once the file has been created, try adding signatures and see if that works.