[Describe what you were doing when the crash occurred.] I opened my Zotero app, it attempted to load the past pdf that I read, and then it crashed repeatedly.
If the app crashes every time you try to launch it, because it tries to restore the last PDF, you can tap and hold the app icon and select the My Library action to launch to that screen instead.
Upgrade Storage
However, we have a potential fix for these occurences in the latest beta, if you can please install and provide further feedback if needed.
https://www.zotero.org/support/beta_builds#zotero_for_ios_beta