Hello,
in the PDF editor of the PDF24 Toolbox, undo and redo only cover the changes on the page that is currently shown. The buttons become active after a change on that page, and the history is cleared when you switch to another page. Ctrl+Z and Ctrl+Y work the same way.
The online editor has one history for all pages and tools: https://tools.pdf24.org/en/edit-pdf
If the buttons stay inactive right after a change on the same page in the current version, please tell us what you changed (e.g. added text, drew a shape), then we will look into it.