Document-scanning workflows, fax software, archives and some print and legal systems expect TIFF, which can hold many pages in one file. Converting turns each PDF page into an image, so the text can no longer be selected or searched.
The defaults suit most files. Under Options you can choose which pages to convert, set the resolution 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. Every selected page goes into one multi-page TIFF file, compressed without loss, rather than one file per page. A password-protected PDF needs its password, entered under Options; without it the file is refused.