It opens PDFs and images. The nice thing is it is super quick and has a lot of features built-in: OCR of text in images, form filling in PDFs, signing PDFs, annotation, cropping, resizing, metadata inspection, background removal, etc for images.

The signature feature is awesome - it lets you use a trackpad or iPhone as signature pad, or capture an image. Signatures are saved for future use.

PDF support is really great - things like annotations work well and it supports proper page numbering (e.g. textbooks often have page 1 starting 10 or 20 pages in; Preview handles this) - something that built in options on windows tend to lack.