Overlaping PDF and Illustrator, in an automatic way

Hi everyone,

I have :

- An Illustrator file (made of colored lines)

- A PDF file, that has an unknown and unnavaible police.

I wish to create a new PDF, that would be made by overlapping theese two documents. So i'll have the colored lines over the PDF file

I can not import the PDF file on Illustrator, because the text becomes hieroglyphes

I can save the Illustrator file as a PDF, but then it has a white background that hides the drawing when i overlap them.

I can save the Illustrator file as a PNG without background, but then i dont know how to overlap it with a pdf.

I have to do this hundreds of times, so if you guys know an automatic way (through scripting maybe) that would be awesome.

Thanks for your help,

Audrey