So you want to quickly/cheaply crop a jpg or pdf but have no tools lying around your mac to do so. There's actually an easy way to do it using Mac's native image and PDF viewer Preview.
Command + K
Really handy way if you can't be bothered installing gimp, or alternative.
- Open the file in Preview
- Select Tools > Rectangular Selection
- Draw your selection area on the file
- Use Command + K to crop the image
- Save your document