evidence for I-140

I am going to prepare I-140 on behalf of my employer( without hiring an attoney),for ability to pay wage, the instructions of I-140 are copies of annual reports,federal tax returns, or audited financial statements,

1.which one ?
or all of them? how many years of annual reports must be included?
2.any other evidence from employer except a letter from the employer? please remind me.



I appreciate your opinions and help.