Sorry if this is somewhere but I can not find "documents for resident visa application"

you mean: permanent residency -which can only be applied for after 3 years uninterrupted stay, three years of work permits and three years of tax returns showing an income of 80K per month?

The Permanent Residency which you can only apply for in December of each year?

That PR?

Info for PR here on immigration website. http://immigration.go.th/nov2004/en/base.php?page=residence

I don't think Immigration actually specifies a salary minimum for PR but B80k per month is specified for citizenship applications (if not married to a Thai - B40k if you are). However, it is presumed that Immigration will use their discretion to reject applicants on low salaries, as not contributing enough to Thailand. So B80k may be a reasonable guideline, as a it is a burdensome and lengthy process not worth the hassle, if you have a low chance of success.

The 80k number is mentioned under 3.2.3 of the ministerial regulations. See:http://immigration.go.th/nov2004/doc/residence/26122546_regulation_notice_en.pdf