Change the contrast, brightness and saturation of the pages in a PDF. The usual reason is a poor scan: faint pencil or old carbon copy that needs lifting, or a grey cast across the page from a document photographed rather than scanned. Raising the contrast pushes near-white towards white and near-black towards black, which is often enough to make a marginal scan legible.
No - it only changes how the pages look. But it is a useful step before OCR, since cleaning up a grey background and sharpening faint text noticeably improves recognition accuracy.
Raise the contrast and brightness together. That pushes the light grey up to white while keeping the text dark. Pushing brightness alone tends to wash out the text along with the background.
It can. Pages are re-rendered when the adjustment is applied, so the result may be larger or smaller than the original depending on the source. Run Compress PDF afterwards if size matters.
Files are uploaded over an encrypted connection and processed on CoonTool's own server, not in your browser. The working copy is deleted automatically once the job is done, and in any case within 24 hours. Nothing is stored permanently and no account is required.