Let me try to answer your questions:
|Can I promote my website with Adwords as an affiliate review website?|
Well, that depends on what you mean by the “affiliate review website”. For many folks, the term “affiliate review website” is just another label for “fake review website”, which is a violation of AdWords bridge page policy.
If your website gives unique, unbiased reviews that are truly useful to people that aren’t interested in your affiliate offers, then it might be okay, otherwise, no.
|Can I promote the landing page of this domain with an email capture page that gives the user to download free pdf guide “how to trade forex” for example in return of his email?|
Generally, no, AdWords doesn’t allow you to offer something free in exchange for personal information. You can, however, ask people to subscribe to your email newsletter and offer a bonus for signing up. It may seem like a subtle difference, but it is a difference that either complies or violates AdWords’ policy.
The distinction is in the purpose for collecting the email if you have a reasonable purpose (subscribing to our newsletter) clearly stated then you can ask for an email, but simply offering free gift and no clear purpose for why you want the email, or what you intend to use it for, is a violation of policy.
|What do I need to be careful with?|
Obviously, you need to be careful of the policies related to your first 2 questions, but not just those policies. AdWords takes each and every one of their policies very seriously, you cannot ignore a single one of them.
In general, I would say do not try to trick, deceive, or abuse your audience in any way. Try to provide high-quality content/products/services in a useful way while following all AdWords policies and you should be just fine.
If you think you might be dangerously close to violating policy, you probably are, so steer as clear from policy violations as possible.
You can find all of AdWords’ policies here: https://support.google.com/adwordspolicy/answer/6008942
P.S. You need to fully disclose that you are being paid to endorse the products or services that are included in your reviews, otherwise, you are likely to be in violation of AdWords’ policies along with FTC regulations against fraudulent, deceptive, and unfair business practices.