Compressing a PDF reduces its file size so it is easier to email or upload elsewhere. You pick a compression level and download a smaller file — your browser rewrites the structure and re-encodes images on your device.
Low compression avoids image recompression; Recommended and High may re-encode images to save more space.
No. If a smaller version can't be produced, the original is kept, so a 0% result is a valid outcome.
No. Compression changes size, not content. Use redaction to remove sensitive information.