Final Cut Pro X 10.3.4 is a capable, stable editor that is best suited for users maintaining older hardware (specifically Macs running macOS Sierra or High Sierra) or those needing to access legacy libraries.
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If you previously purchased Final Cut Pro X when version 10.3.4 was active, you can re-download it via your purchase history. Open the on your legacy Mac. Sign in with the Apple ID used for the original purchase. Click on your Profile or the Purchased tab. Locate Final Cut Pro and click the Download icon.