Preview is a powerful and versatile app that offers a range of hidden features, including digitally signing documents, removing backgrounds from images, adding annotations to PDFs, merging multiple PDFs into one, annotating screenshots, deleting or rearranging pages in PDFs, cropping and resizing images, adjusting image color, opening multiple images at once, reverting to old versions, converting files, reducing file size, and taking screenshots. These features make Preview a convenient tool for editing and manipulating various types of files, without the need for additional software.