Open the file, select the white background using the magic wand and hit delete. It should delete the white space and make it transparent. If you need to select white space that is inside a letter (like the letters 'o' or 'a' etc), you can shift-click with the wand or you can deselect Contiguous and the wand will select everything that is white. Use the tolerance setting to adjust which shades of white will be grabbed.