Say you are modifying the font size for a Purchase Order PDF.

1. Open the Settings module and Activate the developer mode (bottom right)

2. Open the module that the PDF is in. In this case, it is Purchase

3. Activate Odoo Studio

4. Open Reports tab (top right)

5. Open the PDF you are trying to edit

6. Click on Report on the sidebar on the left 

7. Open XML and there is a page with code.

8. There is a drop down menu where you can view multiple layers of the code

9. Look for the report named report_purchaseorder_document

10. Insert the code style="font-size:12px" within any of the <> sections
        <th ____________ /> 
        <span _____________/>

Bonus: 

color: red (changes color of font)

font-weight: bold (changes font to bold)