Adobe recently released a new version of their pdf reader called DC. It appears unable to display the TLA+ Hyperbook in an acceptable fashion. I am in touch with someone at Adobe and will report on what I find out. Meanwhile, if you want to read the Hyperbook, I suggest that you don't update to this version. The previous version is still available on the Web, so you can re-install it if you've already installed the new version. The version I found is 11.0, but when asked to find updates, it let me install 11.0.13--which I believe is the last version before DC.
Leslie