How do I reduce file size in Pages

File menu > Reduce File Size… on Pages for macOS. In a review of the Pages for iPadOS documentation, I see no comparable solution for file size reduction. You may need to open that document in Pages for Mac to reduce its file size. I do not have Pages for iOS/iPadOS installed.

However, that is only part of the story because if you insert several high-resolution (300 dpi) images in the document, you will still need to replace them with lower resolution (i.e. smaller) images to reduce the overall document girth.

You are welcome. The Pages applications do not have identical features on Mac and iOS/iPadOS platforms with the Mac version having more functionality.