How the signing certificates that you use for all signatures were procured? Were they created by a reputable Certificate Authority (CA) or are they self-signed certificates that you can create in Acrobat and other apps?
If they are self-signed certificates then you need to explicitly trust each of them on each Acrobat/Reader installation. You can use (Acrobat XI/Reader XI) to do that:
1. Right-click on the signature, select "Show Signature Properties...->Show Signer's certificate".
2. In the left pane shows the signer's certificate. Select it.
3. Select "Trust" tab on top and in this tab "Add to Trusted Certificates". Again for self-signed certificates you need to do that on each Acrobat/Reader installation.
If you got your signing certificates from a Certificate Authority #2 above shows the certificate chain. Select that top-most certificate in the chain and add trust for it as in #3. Chances are that all your certificates were issued by the same CA. In this case you need to add trust to only this one certificate on each Acrobat/Reader installation.
You can use Export/Import Security Settings feature in Acrobat to make this process easier. This feature was discussed at length in several discussions on this forum <Re: Small Business Digital Signature Set-up>. Search for "Export/Import Security Settings" for more info.
Thank you for your answer.
The certificates are self-signed certificates, so I can do what you explained.
When I do this though, it tells me that "If you change the trust settings, you will need to revalidate any signatures to see the change. Trusting certificates directly from a document is usually unwise. Are you sure you want to do this?"
What does it mean to revalidate the signature?
Thanks for your advise to search for additional info on the Small business section. I shall do that.
Do not worry, this alert is mostly for the "real" (CA-issued) certificates. This is an easy way to set trust for self-signed certificates. Otherwise, you would have to export public key of your self-signed certificate, go to "Trusted Identities" dialog and do the work there. This is just a shortcut which you can use, provided that you understand that you are doing. The warning is for people who don't.