Google and Google Scholar both include lots of material and use algorithms that are constantly being adjusted to highlight the "most relevant" results. Expect good recall, low precision, and little quality control.
When you search in Google Scholar, you'll see links labeled [PDF] to the right of search results. This connects you to full text from many resources: PubMed Central, ResearchGate, Academica.edu, or institutional repositories.
Do the same search in Google Scholar and Google, and you'll get different results. You can limit your Google results to PDFs by adding filetype:pdf to your search string; this can help you track down full text of articles, reports, and white papers.
PubMed and PubMed Central are both free databases that you can search with high precision. To check which database you're using, look in the top left corner. Even though they look similar, there's an important difference! PubMed is a database of citations; PubMed Central is a database of articles.
It's almost always better to look for articles about your topic in PubMed, which is much bigger and updated more quickly. We recommend searching PubMed Central in one particular situation: if you need to search for keywords in the full text of articles.
PubMed has a free full text filter on the results page. Look on the left, under text availability, and click on free full text. The results set for your search will get smaller, but all the remaining articles are easily available in free full text. Just click on the article title to get to the article page, then look for the full text link on the right.That full text link will bring you to the article's full text, maybe in PubMed Central or maybe somewhere else.