On April 12th, Apple began notifying customers running macOS High Sierra 10.13.4, about apps that have not been updated to use 64-bit processes. 32-bit apps now produce a one-time alert when opened. This notification gives users notice that they are running 32-bit software, which will not be compatible with macOS in the future.
Users running macOS Mojave will need to install Bookshelf 8.
Visit Apple’s Support Page for more information on Apple’s timeline for transitioning macOS X software from 32-bit to 64-bit.