Get .p12 and xml files using javascript from the server

I need to call a javascript function that signs a receipt, but I do not know how to get these files and pass them to this function:

var comprobanteFirmado = firmarComprobante(archivo_p12, Clave_p12, xmlDoc); 

Try to send them by php because it is easier there but it gives me errors since php sends them already open as a string. And apparently the function needs them integro.

Agradesco your help.