Why a "unknow signer entity" error is shown on Adobe Reader
Answer
That happens when your Adobe Reader does not have the root CA of the certificate which you have signed. To solve it you should setup the application to accept the CAs from your Windows repository.
To do it, choose on Adobe Reader the menu option "Edit" and "Preferences".
In the following dialog select the "Security" entry -> "Advanced preferences" and tick the checkbox "Enable the search of different certificates of yours on the Windows keystorage.
If you re on Linux you should setup the application to trust on the certificate from the own Adobe Reader trust repository.
Open a PDF document with the signature, click on the top, on the signature button panel. A left frame with the signatures section will be opened, click with right button and choose "Show the signature properties".
On the opened window there is a button "Show certificate", click on it. A new window is opened, on the trust tab click the button "Add to trusted entities".
Accept and close the remaining windows, now when Adobe Reader verifies the signature will appears as valid.
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