Apply digital signature of pdf documents on my website from Digital certificate or electronic DNI (spanish ID)

I currently have a website with which we fill out forms (work orders) and then generate a pdf (using tcpdf) from these data. But now we need that, who fills the form legally sign the document by: Digital certificate or electronic ID (FNMT from Spain). And so, that the generated pdf is legally signed by that digital certificate or electronic DNI. If possible we preffer not to use a Java applet due issues with internet browsers.