It's, imo, a funny question, because as far as I know: 1/ PPC (like Google AdWords) has nothing to do with SEO itself. PPC is paid advertising campaigns; SEO refers to organic results that a SE displays below/next to/under/etc. paid ads.
2/ The organic SEO results may be short or long term and take some certain time, whereas PPC delivers quick results as long as you are paying money to show your ads.
So, there will be no effect of PPC on organic SEO rankings, because the only thing PPC does is to bring
paid content to the top of the SERPs and to show them to people when there is a need for relevant keywords related to that content.
Conclusion: It will affect your ranking neither positively or negatively; doing an ad campaign should not affect your rankings.
If someone disagree, please, let us know and explain what's your point! Thanks!