Mike,
Good question.  I see that AIRNAVs plates are "electronic" since the  pdf is so crisp and clean.  I usually get plates from the AOPA site - these  seem to

be pdf copies of those printed on paper and are much fuzzier but the  collection date ( i.e. SE 1 15 APR 04) appears in the margin.  Also, AIRNAV only provides the airport approach plates while AOPA provides the entire  contents
of the terminal procedures publication including alt mins,  arrival/departure procedures, etc.