"This file can't fit under the limit"

Every format has a floor. A two hour video can't fit in 20 MB and still be worth watching, and a PDF that is mostly text can't shrink much because text is already compact. Okaysize tells you before processing when it knows a file can't fit, rather than handing you a broken result.

What to try

For video, trim it to the part you need, or choose a higher limit, such as a different plan on preset pages. For images, choose a smaller size in pixels. For PDFs, split the document and send it in parts.