Your instinct is sound, that category attracts a lot of junk sites built to farm downloads. The usual markers of a legitimate one: documents readable in the browser without downloading anything, a consistent domain rather than redirects, and no requirement to install a viewer. ManualMachine https://manualmachine.com/ is one I have used without that nonsense, documents open directly in the page. Beyond that, never install anything a manual site suggests, and if a file arrives as an executable rather than a document, that alone tells you what it is. And if a site asks you to register before showing anything at all, that is usually a sign the document is not actually there.