I do not have Acrobat 9 to check but you generally need to have the signing certificate with the private key that signed the signature you want to delete (installed and enabled either in the Windows store or Acrobat Digital IDs) in order to be able to delete this signature.
Open Security Settings dialog, choose Digital IDs and look if you have the signer of the signature. If you do, highlight it, select "Details" and check whether "Details" pane contains private key.
This what happens in Acrobat X and XI. I just presume that the same happens in 9, although I cannot guarantee that.
It's important to keep an archive of files before
- setting security
- reader enabling
These are serious and often irreversible.
1. open file in NitroPDF (U may use Acrobat, but the bookmarks will automatically be lost with Acrobat, but preserved by NitroPDF)
2. Select all pages and click Edit>Extract then select Extract selection
3. Save the resulting "Untitled" file with any name you like eg "Originalfilename"_Extracted.pdf. U have now removed the digital signature!
4. If the file contains "Form Fields" that U wish to remove then...
5. Open the resulting file in step 3 above, in Acrobat
6. Go to "Tool Sets" and select "Forms", then click "Edit Form"
7. A list will show up with something like Preflight. Right click it and select "Remove", then remove anything else that is removable
8. Save your file
9. Now your file is free of both the digital signature and the form fields!