How can merge all a PDF pages to single page only .

Question

I want any advice merge all a PDF pages to single page (as example if I have PDF file in 4 pages I want the same PDF content in 1 page) , all libraries I have found with Google search talk about merge different PDF files and that not my target , please if any one know how to do that in asp mvc5 with free library as I will publish my app for users free incharge so I don't want to pay any additional costs , thanks at all for every one.