Apple - Scaling text in Keynote

My solution:

• duplicate the slideshow,
• resize the entire document (Document tab of the 'properties sidebar' > Slide Size dropdown)
• copy the correctly sized elements back to the original doc.

Any variation of this will probably work - the key principle is that if you resize from the Arrange properties tab, objects will resize whilst text won't; if you resize the document: everything stays in proportion.

Using the accepted answer's method, I wasn't able to edit the content I'd pasted from preview (the pdf contains a rasterized png, I believe).

In Keynote.app: Select all parts of the graphic you need to scale, then copy them to the clipboard (Command-C).

In Preview.app, just create a new document (Command-N). Contents from the clipboard will be automatically inserted. Save this document as PDF somewhere, and drag it to Keynote. This should do the trick.