I would strongly advise you against taking out a loan of this size. To do so would be a great risk to your business.

to 在这里是不定式符号，用于引导动词原形 do。 do 是动词原形，意思是"做、进行"。 so 是代词，指代前面提到的"taking out a loan of this size"（借这么大一笔贷款）这件事。 to do so 这个短语的意思是"这样做"，指代前文提到的具体行为。