This is a loaded question. Each developer and end user will have some bias as to when a Dossier or a Document should be used. There are some general guidelines I have followed on when I should use a Document or a Dossier.
If you have a long history in working with Business Intelligence tools/platforms, you'll be familiar with the accounting style report. Those reports which are ready to be printed and look exactly the same on the screen as they do when they're printed out.
Being able to print pixel perfect reports that look the same on screen as they do on paper is one of the main reasons to use a document. There are other reasons though. Documents traditionally provide more flexibility on design including groupings, sections.
Dossiers are most often used for fancy visualizations and data discovery. If you're more familiar with the Power BI or Tableau visualization style and need that as part of your design, a Dossier is for you. Dossiers provide responsive design and change to fit the device (mostly) naturally. Dossiers are often use to give that "wow" factor which is eye catching.
Which should I use?
That's still a loaded questions but I would make my recommend based on some of the information above.
Do you have anything to add? Write it in the comments below. I'd love to hear from you.