Please be sure that the file is a .tif or .tiff file. TIF Reader only supports these file formats.
iOS 8: If you are in the Mail app, please be sure to enable the Read TIF App extension, or follow the iOS 7 instructions below. You can watch a short video on using the app extension here.
iOS 7: If you are in the Mail app, please be sure to follow all these instructions to open your TIF file from Mail into TIF Reader.