Regarding your concern, I would like to confirm that portfolio only allows images.
- Site users can add links to their pdf files in their description.
- Else, site freelancers can add an image of any front page & add the link to the pdf in the description.
- If you want to include pdf, you will need a pdf viewer functionality of which is not supported by WordPress default feature.
- You may need to consult freelance developers for further solution.
We appreciate your understanding.