Upload your file, set the name to either "High School Intern Program Agreement" or "Internship Sponsor Questionnaire".
Set document category to be other. Type to document or pdf. Leave the description blank.
A common problem students run into is that the file will not upload due to being larger than 5MB. This is a frustrating error, because it is not an unreasonable size for a scanned pdf. Navigator is the worst - I have listed this as Gripe #1. A few possible workarounds -
Try splitting up your document page by page.
Screenshot each page of the document, upload the screenshots.
Use an online tool. If a site asks you to make an account or give an email, don't use this tool.
At this point, if you are working on registering your internship, your next step is to request your internship on Navigator.