The obvious difference between fractional and integer mode synthesizer, is that int-mode synthesizers output frequency that is integer multiple of reference frequency, or in this case, f_PDF. Frac mode synthesizers, on the other hand can provide fraction of the step of that frequency, making it possible to generate virtually any frequency at the output.
But, improved frequency resolution is not the only advantage of frac-mode synthesizers, there is something called phase noise decrease. Phase noise increases as 20log(N), meaning that the higher output frequency will look more messy in the spectrum, to say it technically correct. If, for example reference frequency is 100 kHz, to synthesize 3 GHz at the output, we'd need N of 30000. Output divided by 4 (to 750 MHz) would look like this:
Now, if we employ frac-mode synthesizer, we wouldn't need that low reference frequency nor that large N. Instead, we could keep reference frequency at 19.2 MHz and tune the fraction. For example, in our 3 GHz example we'd need N equals 156 and additional fraction of 0.25. This fraction is defined as F/M. Given that denominator M is fixed at 100, F would be 25 and the output (again scaled to 750 MHz) would look like this:
This is improvement of almost 200 times lower N. Spectrum look much smoother and more pleasant to look at.