The 10X buffer is for the reaction. For example if you wanted to do a 50ul reaction, you would add your DNA sample, 5ul of 10X buffer and water to a total volume of 50ul (45ul - volume of your DNA sample). Then to that reaction mixture you would add the correct amount of enzyme.

Michael J. Benedik Would the DNA sample be your PCR product?

Yes, that would be the DNA in your case. The idea is that 10X buffer needs to be used at 1/10 of the final volume of the reaction mix, whatever that volume is.