[t-c-h]

They would need to purify the DNA (remove all other amino acids and membrane proteins etc.) to amplify it and remove any traces of chemicals used for the amplification.

But A) naked DNA will be exposed to the elements/chemicals which could cause damage/denaturation, B) it would look really odd to find batches of pure DNA without any hairs/cells/other proteins.

Maybe some techniques exist to replicate more believable tissue? Honestly not sure as I'm not too well versed on lab-techniques other than sequencing.

[Metacelsus]

Methylation analysis could tell them apart, since DNA from PCR isn't methylated. But most forensic labs wouldn't do this, and I think it would be very possible to frame someone in this way.