After years of trial and error, here is my solution. I hope it can help you too. They are in a new location now where they are stacked vertical. I first taped them together. I left some sticky tack on the back to help it stay straight, but the tack was not going to be bearing the weight of the posters.
I punched 2 holes in the top corners of the poster. I attached binder clips to the metal trim for the drop ceiling. I opened a paperclip to form an S hook to attach the poster to the binder hook.
These have been hanging with this method for several years and have never fallen down. I hope this could help someone with some stubborn posters. But, please check your fire code to make sure this would not violate any regulations. Do you have any tricks for hanging things? If you do, please share them below.