It's a bugbear of mine – Data Merge can't write to individual or fixed page length PDFs from the data merge panel. This CAN be done if exporting to InDesign files, but exporting to 800 individual indesign files? No thanks!
However, there are workarounds. The first one requires merging to one large PDF and then running an acrobat javascript. See this post on the Adobe forums for the “how-to”:
https://forums.adobe.com/message/4633533
The second workaround uses one of two scripts. It requires merging to one large InDesign file and then running an InDesign javascript. See this post on the Adobe forums for the “how-to”:
https://forums.adobe.com/message/5166720
Both scripts provide an opportunity to break the long file based on a variety of variables. It is worth noting that this solution has more application than simply breaking a data-merged file up into smaller PDFs, but also any large document that has to be broken into smaller segments.
One of the scripts can be found at https://www.loicaigon.com/en/pd…..rly-named/
The other script can be found at https://forums.adobe.com/thread/1014766 (link within the Adobe forum) – this one however is in German
However, it would be nice if data merge just did this without having to resort to workarounds… – sigh – .
Colin