You probaby don’t.  Split it into two documents before it’s converted to PDF. 

It depends on whether or not each piece needs to be separately viewable or they will be appended to each other to recreate the original full file. There are a number of ways to chop a big file into smaller pieces.
 
But turning a large .PDF into a set of smaller coherent .PDFs requires essentially decompiling the full file, separating the data content into the size of pieces that you need, and finally recreating each part as a new .PDF using all of the attributes that were extracted from the decompile step.
 
It wouldn’t really be a “decompile”, but simply opening a few .PDF files in Notepad and reading their internal structures will show you some of the elements you’d have to work with.
 
You might be able to use Adobe PDF products to do the kind of editing needed. And there are probably other PDF editing products that can be tried.
 
Tom