I need to free up space on my iPad. If I delete most of the photos on my iPad will they still remain on my PC?

Then yes, its possible they may be deleted:

Delete photos and videos in iCloud for Windows - Apple Support

What happens if a photo or video is deleted from an Apple device or iCloud.com?

If you delete a photo or video stored in iCloud Photos from your Apple devices or from iCloud.com, one of the following happens:

• iCloud for Windows version 10 or later: The photo or video is also deleted from your Windows computer.
• iCloud for Windows version 7: The photo or video is only deleted from your Apple devices and iCloud.com. To remove it from your Windows computer, you need to delete it from your PC.

Depends on how they got on your PC.

If you are using the iCloud Photo Library, then any changes get synced across devices. so if you delete something from the iPad it will sync to iCloud and down to other devices accessing the iCloud Photo Library and be delete from everywhere.

If you transferred the photos in any other way to the PC, then deleting them from the iPad should not affect the PC.

You can turn off iCloud Photo Library on the iPad, and then delete the pics from the iPad to not affect the PC.

Not unless the J:\Photos\Pictures\iCloud Photos\Downloads is actually syncing to iCloud Photos.

In general, the iCloud Photos folder is on C:\ and cannot be changed directly. It appears as a location directly in Windows Explorer. So if you are physically copying the photos from there to downloads in your J: drive, then they are no longer synced to iCloud.

Set up and use iCloud for Windows - Apple Support