I didn't like not being able to merge .PDFs without spending money or uploading my files to some 3rd party website.I solved this by creating a simple open source windows application to do just that. I'm using iTextSharp and WPF.
Clone the repository and run the .exe inside the "Release" folder.
If you found a bug, or want to improve the program by all means play with the code (or message me)!
Dimitris Tsimpitas 2017