Pubmed Central (PMC) is a subset of journal articles, reports and other materials. Most of the articles will be available in full-text.

Importantly, many reports from the NIH and other government-funded entities, who are required to publish their findings as a condition of receiving funding, will submit them to PubMed Central so that they are publicly available from a reputable source, as required by the funding agencies. 

The PubMed Central Collection of journals are those that meet certain criteria of the the National Library of Medicine (NLM). Often these journals are submitted for consideration by the publishers. Almost all of the journals have their recent issues available in full text inside PMC.

NLM considers the suitability of a journal for their collection based on the criteria in the Collection Development Guidelines, and the recommendations of external, expert consultants. The current scientific and editorial quality review process for journals that apply to participate in PMC has been in place since November 2014.

Adapted from https://www.ncbi.nlm.nih.gov/pmc/pub/journalselect/

Some journal articles or reports in PMC are embargoed. This means that the content has been submitted but is being held back, usually at the author or authors' request. There are many possible reasons for this.

To see these citations, which may or may not have the content attached, go down on the left sidebar to additional filters, and choose additional filters (this view is enlarged to show details). Click on text availability if it's not clicked. Next, scroll back up, and under Text Availability, click on "Include Embargoed Articles" if it's not highlighted. This will retrieve a larger set, but, again, some of them may not be available in full text yet.