The service pack are release to resolve the exisiting Bugs or limitation. Some times few enhancements are also included. With every service pack release, HP also share the "Release Notes" for the corresponding SP.
For HP PPM 7.5, SP7 is the latest available patch. More details about the patch and enhacnment can be found in attached "PPM7.5_ReleaseNotes_SP7" document, shared by HP.
Cheers.. Utkarsh Mishra
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PPM 7.5 has reached end of support life, therefore no further fixes or developments are being invested. There still is a certain amount of time that this version is under self-solve support and as long as that is given, you can download manuals from SSO. You can always, as you now do, try to get assistance inside this forum.
If you want to know about the funcationality changes, enhancements etc of new versions, I suggest you download the guides 'what's new and changed', respective release notes, admin guide and upgrade guide for the interesting versions from the manuals section of self solve online. These will give you a good overview of currently supported PPM-versions as well as their integrations.