...This Libary (or Tutorial ?) is the result, its using IText for C# and it's made to use your physical CPU Cores and do Parallel Execution.
I will make it short, 1000 PDF needs 15 seconds(On a 6 Core Machine with 50% Usage Peeks), also the Filesize is 50% smaller than Adobe and there is no visual difference (if there change compression...)!
This save me a lot of time and since this is for my work also money. Adobe may be great for other tasks but not for simple merging.
PS: The GUI Program its just a Demo, its fully functional ofc and im using it, but as the main "point" of this small Project look at the libary and see how i merge them maybe its useful for usage in other Program's.