Adobe Acrobat keeps crashing and I can't get anything done. What should I do?

I'm on a free trial and there doesn't seem to be a person who can help me. I'm just trying to see if this program can do the things I need it to before I purchase but it keeps crashing before I can edit a document. I don't want to buy this, since it doesn't work. I'm on a Mac. Is there another program that is better than this crappy one? I need to combine PDFs and change the page size before finalizing my PDF. That's it. Adobe Acrobat can't handle it without crashing and I really need a program that can do this for me. Before I spend a dime on Adobe, I thought I'd ask if anyone knows of a better option.