Google's Advanced Search offers some narrowing options including site or domain and file type.
Choose specific keywords.
Consider word order. e.g. dog chow vs chow dog
Phrase searching is supported e.g. "body mass index" "jekyll and hyde"
OR (in uppercase) is supported.
AND is not needed. Google automatically ANDs keywords.
Google does not use parentheses. In short, you can't mimic some of the more complicated searches you can create in many databases.
Keep it simple.
Limit to a filetype with filetype:PDF following your keywords e.g. media elections tampering filetype:pdf
Limit to a domain with site:edu following your other keywords e.g. obesity trends site:gov
Use a word such as data, stats, or statistics to locate this type of information. Pair with other keywords and domain searching. e.g. stats inequities health care site:edu