I am a huge fan of superheroes - Superman, Batman, X-Men; I love them all. So any chance I get to make a superhero analogy I take it, and I think this is one of those chances, so bear with me.
If you are familiar with the world of X-Men, you know that 2 of the central characters, Magneto the "bad guy" and Professor X the "good guy" started out as friends, then were enemies and then in certain situations (like when humans try and kill all the mutants) sort of friends again. So all and all, a very complicated friends/foes relationship. But before I go off the nerd deep end, I'll answer the big question, "Are PPC and Inbound Marketing friends or foes?"
The answer to that is, just like Magneto and Professor X, they can be both. It just depends on the situation.
Here are a few situations where PPC and Inbound Marketing can be "friends":
Situation 1- If PPC campaigns are leveraged to promote Inbound content (blog posts, content offers, etc.) to larger audiences.
Let's say you own a comic book store and create a checklist "5 Steps to Protecting Your Vintage Comic Books". You could launch a PPC campaign promoting the checklist and driving people to the landing page where they would download it.
By launching this campaign, you would be getting the checklist in front of a larger audience than normal, and people who might not have otherwise found your store's website could visit it.
Situation 2 - If PPC tools, like Google AdWords, are used for keyword research.
Not only can these tools be used to help you get a deeper understanding of how your audience finds you, but they can also be used to help you select the keywords that you should be using when you are writing Inbound content and optimizing your site for keywords.
Pro Tip - look for keywords in your industry with high search volume and low competition.
Now let's take a look at a few situations where PPC and Inbound are "foes."
Situation 1 - If your PPC campaigns are only focused on directly promoting your organization and selling your products or services. If you are familiar with the guiding ideologies of Inbound Marketing, you will know that the ideologies of PPC and Inbound might clash a little bit in this situation. However, there can be certain scenarios where PPC campaigns can be run to try and sell products while also running an Inbound plan.
Situation 2 - If your PPC campaigns are only focused on driving massive amounts traffic to your site. This contradicts the Inbound focus of attracting quality traffic (that will eventually convert into customers) rather than large amounts of unqualified traffic to your site.
But overall PPC campaigns can be a great tool to help share your awesome Inbound content with a wider audience and do it using the language that they are using.