While not allowing them to edit or change the main part of the document.
Debby Lentfer
Using the free Adobe Reader DC recipients of the handout manual can use the Comment or Drawing Markup tools to add their own annotations to the PDF. These annotations are not part of the actual PDF page content.
Be well...
If you want to add another layer of security, you can also protect your document and only allow certain things (e.g. adding annotations/comments). Here is a short tutorial that demonstrates how to do that: https://acrobatusers.com/tutorials/protect-pdf-files-with-passwords-and-permissions
You want to add "Password Security" and prevent changes, and only allow "Commenting, filling in form fields..."
Karl Heinz Kremer PDF Acrobatics Without a Net PDF Software Development, Training and More... http://www.khkonsulting.com