PPD is similar to what is called "incentive" promotion. You are asking people to complete a survey/offer in exchange of something they want.
Now, in CPA, there is incentive and there is also non-incentive. Non-incentive ones are promoted like you would promote a banner ad or an adsense ad for example. You cannot promise the user anything in exchange of completing the offer or clicking on ads for that matter. You just talk about the offer, speak of its benefits and then you present them with the offer. You could, for example. make a nice video about an iPhone and direct the visitors to a landing page of a free iPhone 5 zip/email submit offer.
If the same offer was an incentive offer, you could make a video about, let's say, getting cheats codes of "head soccer" and direct them to a landing page where the cheat code is locked by a content locker. They fill out an offer on the locker and then get the code. That's what the PPD networks do, lock the content. However, the PPD networks usually pay you less than CPA networks.
Peerfly is not an incentive network. They have an incentive network though, it's called incent.ly. They don't have their own content locker yet.
Some other good incentive CPA networks are adscendmedia, adworkmedia, adgatemedia, MGCash and they all have their own lockers.