Does Marketo have the ability to send the HTML emails as a PDF attachment to avoid the image errors that Outlook applications display as a security default (unless there is a way to enable the images, the images appear as an “X” in when I opened a sample email sent from Marketo). Our customers will probably delete the email if this is how the default view looks, and have mentioned they prefer PDFs as they are easily printable/shareable.
Marketo does not have support for email attachments, and even if it did, corporate firewalls generally do not accept emails from outside servers (esp. known marketing email servers like Marketo's) with attachments.
Add a link to the PDF file with a "Download" link or button, and make the PDF available that way.
Jason's right. You can also avoid having a big x in your email where the image needs to be downloaded by designing the email with minimal images and just include a link to the PDF of the document you want to share.