How best to conduct user research on existing textual documents?

My team and I are designing an online service that hosts learning material in the form of articles with various sections that take maybe 10 minutes to read.

While we can't change the content, we can change how the content is presented/chunked up etc to make it more digestible for the user.

Is it potentially better to do user research retrospectively (ie. a survey) to understand feedback about presentation of the articles? Or is there a better option?