Attaching a file to e-blasts can be done by linking to the file from the message. Strictly speaking the file is not an "attachment" but a link.
This way you can track who clicked on the link to download the attachment while there is no way to track "standard" attachments.
Select some text in your message, for example "download PDF here" and click the Link icon in the editor toolbar. Then, you'll see an "Upload" button on the next screen just under the URL field where you would enter a website link.
Click the Upload button and you'll have a option to select the PDF from your computer and upload it.