To upload a PDF to your OneNote page:
1. Make sure you're in the app and not the Safari version
2. Create a page in the section of your notebook your teacher has told you to use
3. Name the page by writing a title on the line at the top of the blank page
4. Tap in the white space under the title
5. Click "Insert" at the top of the screen
6. Click PDF (just under the Insert button)
7. Search for your file (you might need to click around a bunch or read this article to guide you)
If the file you need won't let you select it, then it's probably not a PDF. Make sure you're saving your files as PDFs.
You could try Insert Picture or Insert File if the file you need to use is a picture.