In the preceding example, each file that has the .doc or .txt extension in the current directory is substituted for the %f variable until the contents of every file are displayed.
There is no difference between %f and %lf in the printf family. The ISO C standard (all references within are from C11), section 7.21.6.1 The fprintf function, paragraph /7 states, for the l modifier (my emphasis): Specifies that a following d, i, o, u, x, or X conversion specifier applies to a long int or unsigned long int argument; that a following n conversion specifier applies to a pointer ...