Formal:
* Merci de votre lecture. (Thank you for your reading.)
* Je vous remercie d'avoir lu cet article/ce livre/cette lettre. (Thank you for reading this article/book/letter.)
Informal:
* Merci d'avoir lu ! (Thanks for reading!)
* Merci de la lecture ! (Thanks for reading!)
* Merci pour votre lecture ! (Thanks for reading!)
You can also use a more specific phrase based on the type of content you wrote:
* J'espère que vous avez apprécié la lecture. (I hope you enjoyed reading.)
* N'hésitez pas à me faire part de vos commentaires. (Feel free to share your comments.)
Ultimately, the best way to say "thank you for reading it" in French depends on the tone you want to convey and the audience you are addressing.