Perusall integrates with the Intellus search engine for finding Open Education Resources. To use it, go to your course and then click Documents > Add > Open access materials from Intellus. Once inside Intellus, select the discipline of your course and then use the search engine to find a resource you would like to import into your Perusall course. Click Add to Perusall to add this document to your course.
Intellus indexes some resources that are not in a form that be annotated using Perusall (for example, videos). These materials can still be added to your course so that you can organize it along with your other course materials. However, these materials cannot be assigned within Perusall, and when students open one from the Documents tab it will open outside of Perusall in a new window.