You are using Keynote v12.1 correct?
Available space on the current storage location where the Keynote file resides. For instance, let's say the Keynote file is on iCloud Drive, and the remaining storage there was less than the size of the temporary space that would be needed during the export operation, and the final PDF storage requirement. That would crash Keynote. So might removing or disabling a font needed by Keynote.
If you have a partial PDF corpse from the export process, remove it. Reboot your Mac, and do a one-time, press and hold the shift key while launching Keynote to prevent it from loading its last saved (crash) execution state. Now, try your export again to PDF and see if crashes again, or just produces the PDF as expected.