Modal window or a page in a new tab?

I'm working on a responsive website for a big organisation that provides complex products that you need to subscribe to.

Subscribing typically means filling a long form of 5 or more steps. Some steps would require A LOT of explanations for some users, however we can't reasonably fit this much text into the form's page.

As a result, I think there should be a link to a page with detailed info. We can't open that page within the same window otherwise the person would have to fill in the form again from the beginning when they go back (technical constraint).

I see 2 options:

• Either we put a link to a page that opens in a new window
• Or we open that content in a modal window (there will be scrolling)

None of these options seem accessible and elegant.

What do you think? Is one of them better? Do you have any other idea?

Please note that I only have power over a small part of these forms.