You can attach destination content from your Documents library or mix and match content from your Documents to personalized a Custom Document for your clients. 

There are 3 different ways to add destination content: 

A) Attach your own document

B) Create a custom document 

C) Add external content

A) Attach your own document

Under Step 3: Documents, select +Add a Document to add an entire Document from your library.

B) Create a custom document 

Select Create A Custom Document to personalize a new Document. Go to Add Existing Pages to search and add content from your existing Document library or Add a New Page to add new content.

C) Add external content

Looking for 3rd party curated content? You are now able to search and add curated content from trusted sites using Add External Content.