More information on Canvas's auto-open inline preview is available in the Canvas Guide "How do I set the auto-open for inline preview for Files using the Rich Content Editor?" Assuming you’ve already uploaded your PDF to Canvas via the Files tool, here’s how you embed a PDF:

1. Go to or create the Canvas Page you want to embed your PDF in.
2. Put the cursor in the page where you want to embed the PDF.
3. On the right hand side of the page where it says Insert Content into Page click on the Files tab. Navigate to where you uploaded your PDF and click the PDF. It will insert a link to the PDF on your page:
   
   
4. Select/highlight just the pdf link in the page. Make sure you don't select any additional space before or after the PDF link. Click the “link” icon :
   
   
5. A Link to Website URL window will open. Check the box next to Auto-open inline preview for this link and click the Update Link button to close the window:
   
   
6. Click the Save button on your Canvas page.
7. The page with your PDF preview should display: