A possible duplicate of another question please check this answer.
This might work for you (but requires advanced UI based customization of Odoo):

Create a new field on the Product model (in this case the Product Variant):

..etc

Happy to help :) an upvote will be awesome

Hello

I noticed the 'deliver content by email' in the Sales app settings. With this, you can reference an email template from the product (accounting tab), and on the email template you can have attachments, e.g. product specs, media etc.. How it works, when the product is invoiced, it automatically sends the email to the client with the attachments. So you would need an email template per product to have product specific attachments. 

If you use the 'upload document' scheduled activity, and link it to a Document app workspace, when you set it on the product, and do the upload, it uploads to the Document workspace and is also accessible from the product. When clicking on the document in the workspace, on the properties pain, it is linked to the relvant product.

I hope this helps!

Alex

Hello Alessandro, 

In order to attach document to a product; you need to select that attachment. 

then on the right side you will get multiple options

Inside this option you need to choose the workspace "Product". Refer the below mentioned screenshot:

https://prnt.sc/rijqde

Thanks

Anisha Bahukhandi

Technical Content Writer