For viewing digitally signed PDF documents, we recommend using Adobe Acrobat Reader or Adobe Acrobat DC.
What are certificates?
Digital signatures via Signhost are secured with a digital certificate that is recognized as trusted by Adobe Reader. This certificate is issued by a recognized Certificate Authority to which Signhost is affiliated. To ensure security, certificates are updated regularly. Therefore, it is important to always use the latest version of Adobe Reader together with the newest certificates. When everything is up-to-date, the visible signature on the PDF document will be validated and considered valid.
Signhost always places a visible signature seal on the document with each digital signature, alongside the underlying (invisible) digital certificate. This seal confirms the authenticity of the signature and can be seen by users.
No signature seal visible on the PDF document?
Then there may be an issue with the original document. In that case, check whether the PDF was properly generated and complies with our PDF requirements.
Why is this important?
Validation of a digital signature is crucial to guarantee authenticity. Technically, it is still possible to alter a digitally signed document afterwards, but Signhost’s process ensures that if a document is changed after signing, the certificate is declared invalid by the PDF reader. This immediately shows that the document is no longer valid. The digital signature remains visible, but the certificate is marked as invalid in Adobe.
Figure 1 - Example of an unknown or invalid certificate in Adobe Reader.
Figure 2 - Example of a valid certificate in Adobe Reader.
Keep Adobe Reader up to date
When opening a document signed via Signhost, Adobe Reader may report that the placed signatures are not valid. This can happen because the certificates within Adobe have not been updated to the latest version, and the issued certificates are not recognized.
Follow these steps to update the certificates:
Open Adobe Acrobat Reader.
Go to the menu > Preferences.
Scroll down the list of categories to ‘Trust Manager’.
Click ‘Update Now’ under the AATL and EUTL settings to refresh the certificate lists.
Restart Adobe Reader and open the document again.
If the message that the certificate is not trusted continues to appear, there may be a problem with the document. If the message shown in figure 2 appears (certificate is recognized), you can trust that the signatures are valid.