The "Assessment Key with PDF" option (under "Create New" in the Lessons/Assessments tab) is meant to allow a PDF to be used for testing on student computers.
Note: If you simply wish to attach a file to a lesson for the convenience of teachers to print a hard copy of the test, or a file that will be used as a supplement to a lesson, etc., please attach it via the Lessons/Assessments>Actions>Attachments feature.
In the "Assessment key with PDF" feature, a PDF of your test questions is uploaded and you enter in the key to match it.
Then, the lesson can be activated choosing the "Engage & Learn" or "Activate for Google Classroom" options in your Lessons tab under the lesson's "Activate" menu.
Several testing options are available like "Scramble Test Questions per Student" and others.
Students can access the test on their computer in a web browser from their Google Classroom account or by going directly to students.allinlearning.com. (Note: This feature is not available in the Engage & Learn app for iPad.) The PDF will be displayed on the left side of their screen and they can enter their answers on the right side of their screen.
To learn more about testing using the Student Portal and Engage&Learn, see this article.
Note that if your assessment has slides with on-screen questions (such as a custom lesson you've built in our PowerPoint-like interface or questions chosen from an Item bank), attaching a PDF here will cause ONLY the PDF to be able to display to students. Question slides will not display.