Convert to PDF

This might sound like a stupid question so bear with me!
Im looking to sell digital content online but when I try to convert the jpg to a pdf it shrinks it to a tiny size.
Im looking for it to be A4 size when printed so is there any reliable software that will convert an A4 jpg to an A4 size PDF?

It's simple, just copy the full-size image in MS word and save it as PDF.

Brilliant! That worked, cheers mate.

I might have figured that out if id had enough time to think about it. Thats my story and I'm sticking to it! lol