1. Copy the document ID. For example, if the URL is https://docs.google.com/document/d/16Asz8elLzwppfEhuBWg6-Ckw-Xtfgmh6JixYrKZa8Uw/edit, then the ID is 16Asz8elLzwppfEhuBWg6-Ckw-Xtfgmh6JixYrKZa8Uw.
2. Open this URL: https://docs.google.com/document/u/1/d/poop/revisions/load?id=poop&start=1&end=1 (replace poop with the ID from the previous step). You should see a json file.
3. Add 0 to the end of the number after end= and refresh. Repeat until you see an error page instead of a json file.
4. Find the highest number that makes a json file instead of an error page appear. This involves repeatedly trying a number between the highest number known to result in a json file and the lowest number known to result in an error page.
5. Download the json file. You might need to remove the characters before the first {.

I found the URL format for step 2 here:

https://features.jsomers.net/how-i-reverse-engineered-google-docs/

I am working on an easy way. Edit: here it is https://lemmy.ca/post/21281709