If you are receiving a message saying "Signature Validity is UNKNOWN" this must be because the Digital ID for that Digital Signature has not been added to the list of Trusted Certificates in Foxit PDF Editor.
To correct this error, verify the signature you are trying to validate is a Trusted Certificate, to do this, go to the Protect Tab then, select "Trusted Certificates."
Make sure the Name of the Digital Signature you are trying to Validate in in that list. If it is not, then simply click on "Signature Properties" (as shown in the first screenshot), from there click on "Show Certificate" and finally "Add to Trusted Certificates"
Going forward, Foxit PDF Editor will Trust the Digital ID added and automatically Validate it for Digital Signatures.
If you'd like Foxit PDF Editor to automatically Trust Digital IDs, follow the steps in the article "How to Automatically Trust Digital ID for Digital Signature"