While you can't embed a PDF in your mailing, there are a few options when it comes to including your PDF in your mailing.
The first option is to upload the PDF file into your document library and link to it. Linking to the PDF has the added bonus of allowing you to track who clicks on that link.
The second option is to send out the PDF as one image file. This is doable, but there are some drawbacks: If you save the PDF as one large image file, this can affect your delivery rates, since many servers will quarantine or reject emails coming through with only images and no text. It can also be difficult to read since there is a limit to how wide the image can be. But you can try saving your PDF as a jpeg or gif file and adding it to an image placeholder. Each image can only be linked to one place, so if you're sending out one big image file, only that image file as a whole can be clickable. So you aren't able to link text to different places, such as a website or an email address.
The third option is to recreate the PDF in the editor by simply creating a layout that works for you, then filling in the text boxes and image placeholders. This is basically a workaround, i.e. trying to simulate the look of the PDF be creating a new email.