Thanks to a blog post from Steegle.com about embedding PDFs into Google Sites, we can use the same idea to embed a PDF into Schoology.
A quick word about copyright. If you are working with a PDF that you've created, copyright doesn't come into play. However, if you've gotten the PDF from elsewhere, the content may be copyrighted. Check the Fair Use guidelines to be sure that you're modeling a responsible and respectful approach to someone else's intellectual property for your students.
First, you need to have the PDF saved into Google Drive. (If it's saved on your computer, you can use the upload option in Drive to upload the PDF.) See the embedded *PDF below (see what we did there?) for the steps on embedding a PDF into Schoology.
After inserting the PDF, it should be visible in your Schoology item (complete with a scroll bar if the length exceeds the default size).
*originally created in Google Slides (view original)