The full-text search within publications listed in Monocl Professional is possible through a specific subset of Open-Access PMC articles available in the platform. This subset is labeled “Creative Commons" (CC). These CC articles are the ones available for commercial use and are therefore implemented in Monocl Professional. The CC subset corresponds to about 85% of all the Open-Access publications; the remaining 15% of the publications will still be listed in the platform but the search will not scan the full text version of the articles, only their title and abstract (when performing a free text search).
Through this link https://www.ncbi.nlm.nih.gov/pmc/journals/ it is possible to have the full list of PMC Journals.