Here is the press release of STM about the acquisition of Lean Library.
This brings us to the following overview of one-click-pdf-tools:
- Kopernio (now acquired by Clarivate; free; shows accessible fulltext for your library (if you have a off-campus solution), plus open access and alternative versions.
- Unpaywall (by Impactstory ; free; brings you open access publications “only”
- Anywhereaccess (by Digital science
- LibKey (by Third Iron)
- Lean Library (now part of SAGE not really a one-click-pdf tool, but assisting to get there anyways)