You can upload pdf for a Magento product with the help of Product Attachments extension:

Step 1. Navigate to Catalog → Products → choose the needed product;

Step 2. Expand the Product Attachments section;

Step 3. Click on the Add New Files button to upload new pdf from the computer or click on the Add Existing Attachments button to add already created pdf attachment.

Here are the few more resource to help you get most out of this Magento 2 Product Attachments Extension.

• How to Configure Product Attachments Extension in Magento 2
• How to Attach or Upload Product Descriptions File In Magento 2
• Build Customers Trust with Our All new Product Attachments Extension for Magento 2