A PDF is hard to edit, and many upload forms and image tools want pictures instead. WebP is smaller than JPG or PNG at similar quality and keeps transparency. PDF is at home in documents that must look the same everywhere, and printing; WebP is the usual choice for images on websites. The result is a picture of each page, so its text can no longer be selected or searched.
One PDF can produce several WebP files (one per page); when it does, they are offered together as a ZIP download. The defaults suit most files. Under Options you can choose which pages to convert, set the resolution, set the quality and set or supply a password.
Pages are drawn at 150 DPI unless you pick another value in Options (up to 600); at the default a US Letter page comes out about 1275 by 1650 pixels. A page that would exceed 16 megapixels is drawn at a lower resolution instead. A password-protected PDF needs its password, entered under Options; without it the file is refused.