Click the Customize button in the bottom-left corner. Ensure the following components are checked:
If your goal is simply to on 10.5.8, skip QuickTime 7.5.5 and install VLC 0.9.10 (last PPC Leopard version). It plays more formats and is easier to find.
https://support.apple.com/downloads/DL923/en_US/QuickTime755.dmg
Before you search for a download link, it's crucial to understand why is the correct version for 10.5.8 .
For just watching videos on 10.5.8, VLC 0.9.10 or MPlayer OSX Extended are often better. But if you need QuickTime for specific workflows (screen recording, exporting, or Pro tools), 7.5.5 is your best bet.