Preserving photos from a desktop MAC

I have roughly 20k photos from the past 15 years stored on an old Apple Desktop. I’m worried it is going to go kaput (software updates not available anymore). I think a complete copy of them is in the iCloud, but to be safe, I’d like to make a copy onto an external hard drive as well. What is the most complete and efficient way to make high-quality copy of 20,000 photos from a desktop to an external hard drive? And what format should they be stored as… pdf? Is pdf the default format of Apple? Thank you!