Use the site: operator to find higher quality websites. Here's how:
Type site:gov OR site:org OR site:edu and add your search terms to make Google only give you back results from government, organizational, or college/university sites respectively.
Ex. site:gov global economic trade agreements
Use the filetype: operator to limit your results to a certain kind of file. Here's how:
Type filetype:pdf OR filetype:ppt then add your search terms to only get PDFs or PowerPoint files respectively.
Ex. filetype:pdf global economic trade agreements