How do I purchase Quicktime Pro for PC?

https://discussions.apple.com/docs/DOC-6562#quicktime - document discussing QT for Windows

You can't. Apple in fact recommended removing any existing installations of Quicktime for security reasons.

You can’t. Apple stopped selling QuickTime Pro keys.

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They’re no longer available for Mac OS X either.

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This web site tends to focus on Apple products so you may not get the most authoritative responses here. Recommendations will also depend upon exactly what you want to do.