Win 10 has a pdf printer that will print to a file it's called "Microsoft Print to PDF" or as stated above you can download a number of free ones.
When you print in GXDeveloper just select the main program (Note: there is no way of just selecting parts of the Main program (there is a way if you have more than one program but if you only have "Main" then you have to print out the whole thing). so your only way is to print both copies i.e. original & manually compare it with the original, you can compare the old file with the new, however, it shows only the difference in statement list (compiled code) and is not printable (as far as I'm aware), also any extra contacts you put in will shift that program down so in actual fact most of the code will show differences, for future programs it makes sense to add extras into separate program blocks, then in PLC Parameters add the extra programs to be called in scan. so I'm afraid your only choice is to print out both the old & the new, most experienced programmers will document the changes in the comments for example, if you add a ladder rung then the comment will state the date and information on what the changes are etc.
The only other way is as above then go through the two programs and use highlight pens on the changes
Go to File\Printer setup , there you can change the printer so if you have a pdf printer installed then you select that as default
