Hi @Ed_,
Here's a sample app that I created with a sample PDF: https://jsapi.maps.arcgis.com/apps/instant/portfolio/index.html?appid=74cc8bcdf20e4bda9012e2e2b54032...
From the Content page in https://arcgis.com,
1. Click 'Create app'
2. Select 'Instant Apps'
3. In the Instant Apps Gallery, click 'Choose' on the 'Portfolio' gallery item and enter app information.
4. In the Instant Apps configuration page in Express Step 1, click 'Add new section'.
5. Select the 'File' option.
6. Click 'Choose File' and select the PDF you are looking to add as a section.
7. Click 'Done'
8. Observe the app preview, At this point, it should display the Portfolio App with the section that was added.
9. Click 'Publish' > 'Confirm' > 'Launch'.
Please let us know if these steps work for you.
Best regards,
Ryan