I agree with Dr.Vladimír , the perfect answer for your question. And I advice to read this book" Physical Principles of Electron Microscopy" by Ray F. Egerton.
In general, the lower accelerating voltage means the higher contrast because more electrons interacts with the sample, compare to higher voltage. In case of samples with low atomic numbers the lower voltage is necessary. For the same reason you can get the better signal gain for EDS analysis.
On the other hand, with voltage lowering the transparency thickness of your sample decrease.
I agree with Dr.Vladimír , the perfect answer for your question. And I advice to read this book" Physical Principles of Electron Microscopy" by Ray F. Egerton.