Digital Signature for PDF files through Web Application

We are developing a Web Application for manage dental clinics.
The app can generate PDF files through the PHP backend (for example invoices, quotes etc.).

Each of those PDF file needs to be digitally signed by both the app user (the clinician) and the patient.
The sign we are looking for is the Legally Signature European Standard (which requires biometrical parameters or a certificate issued by a trusted provider such as Infocert (but we are not 100% sure how to then use such certificate)).

On average, in our country, each patient signs 5-10 documents and because of this we ruled out third party services such as Docusign, Pandadoc, Eversign.. because the average cost of those api services would be excessive for our needs (and country economy).

Another solution we did think about is to use the signature pads (such as Wacom’s) that are capable to collect biometrical parameters, but we have no idea how to integrate such devices with our Javascript frontend (assuming it is even possible to interface those devices through the web browser).

Assuming our backend displays the PDF already generated to the client, what would it be the best approach to apply a digital signature to it through the client (which would then send back the signed PDF to the server for storage) ?
Is there a way to interact with signature pads devices through the browser (to extract the biometrical parameters together with the visual signature) ?

Thanks

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Author: Nite