Lossless and lossy compression
Making a file smaller without losing anything is only possible when there is repeated or unnecessary information: certain metadata, surplus pages or badly packed data. In most cases, cutting the size a lot means discarding detail the eye barely notices, and that is lossy compression. This is why promises to shrink any file to a fraction without losing quality are misleading. What helps is knowing which part of the file is touched and choosing how much detail you are willing to give up.
PDF: text stays, photos change
A PDF is usually heavy because of the photos inside, not the text. Compress PDF recompresses embedded photos and scales down oversized ones, while text and vector graphics stay the same. The gentle level keeps the best quality, balanced suits email and the strongest favors size. If a document has no heavy photos it will not improve, and the tool says so. To shrink a PDF without touching quality, remove what is surplus: Delete PDF pages and Split PDF copy pages as they are.
Photos: pixels, quality and format
A photo’s weight depends on three things: how many pixels it has, what quality is applied when saving it and the format. For a form or an email, a photo 4000 pixels wide rarely adds anything over one 1600 wide. Resize image reduces pixels while keeping proportions; Compress image adjusts quality and exports as JPG or WebP. At high quality values the difference is barely visible; at low values blocks appear in gradients. Always keep the original.
400 × 200 px quality high > low
Videos: resolution, level and container
A video weighs what it does because of its length, resolution and bitrate. Compress video re-encodes at the level you choose, so some detail is always lost, especially with motion or small text. Dropping the resolution to 720p usually saves a lot with little visible loss on a phone screen. By contrast, trimming, splitting, removing audio or changing container copy the tracks without re-encoding when the codec allows it, and then there is no quality loss.
How to compare before sending
Always compare against the original, not your memory. Open the new file on the device or app where it will be viewed, zoom to 100% and look at areas with small text, gradients and edges. Compare the size on disk of both files, not only what the tool shows. If the visual difference is minimal and the size clearly drops, you are done; if not, go back to a gentler level. Keep the original somewhere safe: compression cannot be undone.
Compare two quality levels
Download image-400x200.png and open Compress image. Export one copy as JPG at high quality and another at low quality, and save both. Compare the size of the two saved files and open them at full size.
400 × 200 px quality high > low
Both copies keep the 400 by 200 pixel dimensions and the low-quality one weighs less. If the original is already very simple, the visual difference between the two will be minimal.
Always compare on disk and at actual size, and note the lowest quality you find acceptable. Apply that value to your real photos and keep the originals.