What are digital certificates and how do I keep them up-to-date? 

To view and/or edit PDF documents, we recommend using Adobe Acrobat Reader and Adobe Acrobat DC. This article will refer to the available Adobe tools.

Digital signatures from Signhost are sealed using a certificate, and are therefore seen as trusted by Adobe Reader. This certificate is issued by a ‘Certificate Authority’ to which Signhost is affiliated. To ensure security, these certificates are updated from time to time, which is why it is always important to use the most recent version of Adobe Reader in combination with the most recent certificates. If everything is up-to-date, the signature on the PDF will be validated and trusted. 

The key to a validated signature is its authenticity. But digital documents can still be modified even after they have been signed. The process used by Signhost means that if a document is edited afterwards, the certificate is declared invalid by the PDF reader, providing confirmation that the document is also invalid.
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Occasionally, opening a signed document on Signhost will produce an error message saying that the signature is invalid (Figure 1). Most probably the certificates have not been updated to the latest version, meaning they are not recognized. To fix the problem, update the certificates.
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Open Acrobat Reader > Edit > Preferences

Step 2.

In the list of Categories, scroll down to ‘Trust Manager’ and click ‘Update AATL & EUTL’.

If necessary, restart Adobe Reader and reopen the document. If the notification from Figure 1 is still showing, there may be something wrong with the document. If the notification from Figure 2 appears, you can be sure that the document and the signature(s) are valid.