Una firma numérica o digital.
What can mean "numeric signature" here:
"On this new page you’ll be able to upload in a compressed file (zip, Rar…) your HD file and your numeric signature."
...and what's the translation of these (I don't understand the part of "initial each page":
Please be sure to fill in all of the fields as well as initial each page for both the scanned and physical versions.
This refers to the printed pages in which you have to write your initials (iniciales) on each page. i.e.: Your name is Average Joe, so you must wirte AJ on each page.