Compress to 100 KB is the target a lot of upload forms print next to the file button. This page starts in target mode at 100. It tries higher quality first. If the file is still over 100 KB at the low end, it shrinks the dimensions and tries again. A PNG that cannot hit 100 KB becomes a JPEG, and the note says that happened.
The quality slider re-encodes a JPG or WEBP at the percent you pick, and writes a PNG again when the file is a PNG. PNG has no JPEG-style quality step in the browser, so that slider does not change a PNG. Target-size mode binary-searches quality, then shrinks the long edge if even the lowest quality is still too big. A PNG that cannot land under the target is saved as a JPEG, and the note says so. Strip EXIF is on by default. Re-encoding already drops metadata; unchecking the box copies a JPEG EXIF segment back onto a new JPEG when one was present. It cannot put JPEG EXIF into a PNG.
100 KB means 100 × 1024 bytes. The number under the preview is the file you will download, not a rounded wish.
Before and after sizes are the real byte counts, and the previews are the pictures the browser produced. A batch downloads as a ZIP. This is not compress PDF. A PDF redraws pages and makes text unselectable. A photo should stay on this page. The file converter changes format without chasing a kilobyte target. A target is a best effort: if the picture cannot get that small without becoming a speck, you get the smallest attempt and a sentence that says the target was missed. The page does not label a 400 KB file as 100 KB.