From the website:
ShortPixel Google Drive Optimizer is a powerful tool that compresses PDFs and images in Google Drive to free up more space while letting you keep all your content.
With ShortPixel, you can shrink all your drive files in just a few clicks.
Images and PDFs in Google Drive are automatically downloaded and reuploaded as a smaller-sized version.
ShortPixel even optimizes images and PDFs in folders and sub-folders in the process, so you don’t have to manually reorganize everything, either.
Best of all, the appearance and quality of visual images won’t change. In fact, most photos can be compressed by upwards of 70 [percent] without visible loss in quality.