@ThorX89 - you need to timestamp your signatures. This will solve your problem.
Basically you use a valid (at the time) certificate to sign the PDF, this is timestamped by a third party CA (most Certificate Authorities allow you use of their timestamp servers - check with your provider) and the time stamp itself is verified by a certificate issued by the CA.
In Adobe Acrobat (or Reader) Edit > Preferences > Security > Check Require Certificate revocation Checking to succeed whenever possible during signature verification - then in the box below chose the Secure Time (timestamping) option.
As long as your certificate is valid at the time of signing and the timestamp server is set correctly in Adobe, even if your certificate later expires or is revoked, Adobe checks to see if your certificate was valid at the secure time (i.e. timestamp) of the certificate as certified by a third party trust provider.
To set the time server in Adobe go to Advanced Menu > Security Settings and then on the left you will see time Stamp servers. Enter the New time stamp server and adobe will download a certificate from their server and that will be used to verify the time stamp.
Keep in mind, your Certificate issuer AND the timestamp server's CA both (in case they are different issuers) have to be on the Abode Approved Trust List (AATL) for their to be absolutely no signature verification errors presented to a viewer of the file when they open it in Adobe Acrobat 9.0 or newer. If the issuers are not on the list, they will be presented with 'cannot verify signature' even if it was timestamped. The only solution to this is that (yours and the timeserver's) certificates have to be trusted by the viewer in their Adobe software. Going for a AATL listed provider gets around this problem (for Adobe products).