pdfs on PMC result list

various refactoring

second refactor
Merge pull request #805 from proquest/pubmed-central-pdf
pdfs on PMC result list
[PubMed Central] Minor cleanup. Update tests (formatting only)
[PubMed Central] Add test case for PubReader format
Note that "β" in the title is replaced with "?". This is coming directly from the API. Reported the issue upstream.