Alberto Martin-Martin et al. have self-archived "A Two-Sided Academic Landscape: Portrait of Highly-Cited Documents in Google Scholar (1950-2013)."
Here's an excerpt:
Since the existence of a full-text link does not guarantee the disposal of the full-text (some links actually refer to publisher's abstracts), the results (40% of the documents had a free full-text link) might be somewhat overestimated. In any case, these values are consistent with those published by Archambault et al. (2013), who found that over 40% of the articles from their sample were freely accessible; higher than those by Khabsa and Giles (2014) and Björk et al. (2010), who found only a 24% and 20.4% of open access documents respectively; and much lower than Jamali and Nabavi (2015) and Pitol and De Groote (2014), who found 61.1% and 70% respectively.
The different nature of the samples makes it difficult to draw comparisons among these studies. Nonetheless, the sample used in this study (64,000 documents) is the largest ever used to date.
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