ADE constantly crashing

I prefer to buy my books for my study in digital editions. Therefore I downloaded ADE because it claimed to be the best to read AND make notes at the same time. It also seems to be the only program that supports a png-file. 

But it has become SO slow - and constantly crashing. Does anyone have any advice? I have tried to reinstall the program several times and even downloaded an older version without any luck.

Are there any alternative programs? I can't seem to find anyone. I have a Macbook Air, by the way.

Thank you in advance.

Maria