To render very large PDF document, please change 
the Platform target to 64-bit, rebuild project 
and run the 64-bit targeted exe.

You need to register the 64-bit ActiveX DLL first 
to run the 64-bit targeted exe, run regDLL-x64.bat 
from the OCX directory can register it for you.

Right click the C# project (not the Solution), 
then click Properties, click Build on the left 
panel, select x64 from Platform target option.