When I have to split a file, I do it in this way. I create a backup with a new name. Then I use File > Save As to save as many different files as I want. I give each new file a name that represents that branch of the family.
Then I open the first new file and delete the branches that do not belong. To do that, I click on File > Export > Partial > and select the branches I want to get rid of and click the Delete button to remove. You can also use the Advanced Focus Filter to select the branches you wish to eliminate, and then click the Delete button.
The original file remains intact as an additional backup in case something goes wrong.
According to FamilySearch knowledge base document "2381p_enMaximum records or notes allowed in Personal Ancestral File"PAF 3, 4, and 5 can have "up to one million records"
(But some users report that PAF slows down as the database becomes really large.)