The forgery skill is an opposed skill; it has no base DCs.
When making a forged document, you set the DC of the Forgery check (yes, Forgery is opposed by Forgery) of anyone to detect it. Apply the modifiers for document type and available samples as appropriate.
When someone examines a forged document, they make a Forgery check to detect it, modified by their own circumstances.
The DC to make a document that the town guards will never detect is 21~25, depending on how well they know the document type and how much attention they pay (assuming a guard has int 10 and no ranks in Forgery). But, since you never know how good your documents are, you can't reliably do that unless your modifier is near that high. On the other hand, if you have a +19 (including the +8 for a generic document), they'll only detect you if you roll a 1 (DC 20) AND they roll a 20, so you can get away with less with just a little risk.
--
gnfnrf