Hello @KenF510 ,
PDFs are document-type files. Therefore if you're trying to insert a PDF, you'll need to make a document link.
If you're trying to insert your PDF like an image, we make an image of the first page by default when you upload a document. You may need to refresh the screen in order to see this image available in the left-hand Images tab. If you need a multi-page PDF turned into images that you can insert into your email, you'll need to use an outside program to convert all those pages to individual images.