Please see the reply in the posting:
https://store.eadocx.com/forum/2-eadocx-queries/740-divide-activity-diagrams#740
Sadly, nothing has changed. We’ll see if there is any new support in EA V11, but I’m not hopeful.
Assume for a moment that Sparx provided an API for eaDocX to get the diagram in smaller pieces, how would you like eaDocX to print the pieces? So if your diagram is 3 pages wide and 2 pages long, should eaDocX just print each page, on after another, with a figure number on the last eaDocX page ?

With my Business Analyst hat on, I would still recommend that whilst big diagrams might help to BA to check that the end-to-end process/class model etc is OK, it less helpful to the reader, who probably wants to think about the problem in smaller pieces. So I always suggest that BAs create more & smaller diagrams.
..none of which solves your problem…!