How do I change the font on a 400-page iMac Pages document?
For a quick change up or down in size, select the text and choose Command? and + or -
If I’m understanding you correctly, you would select all the text in the document and choose Format > Fonts > Show Fonts (or Command+T). This will open the Font app so that you can change the fonts.
In Pages ‘09, you can choose View > Show Format Bar, which will put Font selections in Page’s toolbar.
And finally, if your document includes different font styles in your document (e.g. titles, subtitles, body text) use the Styles Drawer to help you make changes to the different styles.