Merging Powerpoint Slides.

Hi,
I have a scenario wherein, based on the user selection of products i need to fetch slides from presentations and merge it into a single file and then give an option to download it.
 
I am already storing the path of files in database along with each product details.
I can modify the presentations in a way that each product has one presentation which can be easily merged without even going to a particular slide of a presentation or have a complete single presentation of each segment of product and a combined presentation for each segment which would require to fetch a specific slide number.
 
I am mainly hung up on the merging part.
 
Thanks in advance!
 
deeZ