The calculated distance (air line) between Saalfelden am Steinernen Meer and Donaustadt is approximately 177 miles respectively 284 kilometers.
By car or train, the actual journey to Donaustadt is certainly longer, as only the direct route (as the crow flies) between Saalfelden am Steinernen Meer and Donaustadt has been calculated here.
Air line (approximately)
284 kilometers
153 nautical miles