Title pretty much says it all. I'm interested to know what are the best practices to make sure the user won't get lost in the gigantic volume of information. There are some basic ideas like a summary or a progress bar but what else can be done to help?
A lot of information will probably mean different things depending on what the user intent is.
A user who lands on your article or case study from a google search will be looking for something specific, you probably know what the keywords are, so having a summary on top is very useful. If you consider SEO best practices then having users land on a content pillar page would be ideal too, this would mean having one pillar page for the topic and break it into derivative sections. This is usually done for reports or guides but can be applied to any problem related to significant volumes of information.
Alternatively, if your users aren't looking for anything specific while navigating your page, they will probably be trying to find reasons to invest their time in reading your content, I believe this will matter more to them than the length or volume of the information. Organizing your article with strong, clear headings, and highlights of content will make it easier to scan, but what will push them to dig further will be the value they get from every minute they spend on it. Make sure to offer enough incentives for them to take that step. Find that 20% that will justify reading the remaining 80%.