What's the difference between Export as PDF and Print, Save as PDF in Apple Pages?
In short: Not much. The only significant difference between the two is the different options that are displayed in the dialogs in each process.
Export PDF gives you image compression, the ability to include various metadata.

&quot;Save As PDF&quot; gives you options to set keywords such as Title, Author, and lets you set Security options.

There may conceivably be slight differences in the way that the PDF is formulated - it's likely that using &quot;Save As PDF&quot; from the print menu will involve the print driver for the selected printer in some way. (Though, for example, note that a mono printer still produces colour PDFs.)
It's all just a conversion from CoreGraphics (which is conceptually similar) to PDF. Apple's PDF-creating routines don't have the latitude and range of options that you find in GhostScript or Distiller.
Comparing two PDFs created by each of the two methods reveals that the data is substantially the same for the vast majority: the differences lie in things like decimal precision of page sizes (again, likely due to page definitions in the print driver).