The design is simple , we need a WEB example using ITextSharp and .NET Framework or .NET Core to sign PDFs using certificates installed on machines ( .pfx or .cert ) the User or SmartCard connected to the USB User 's Machine .
Software flow is as follows .
The User selects the PDF.
And click the button to sign the PDF.
At this time should be requested from the User to select the certificate to be used .
After selecting the certificate and the User enter his password , the site will post the PDF to the server and will be signed when the subscription is completed the server will return the signed PDF to the User .