NLM produces a baseline set of MEDLINE/PubMed citation records in XML format in December of each year. NLM also produces daily update files that include new, revised and deleted citations. Downloading data from the National Library of Medicine FTP servers indicates your acceptance of the following Terms and Conditions: No charges, usage fees or royalties are paid to NLM for this data. https://www.nlm.nih.gov/databases/download/terms_and_conditions_pubmed.html
Yearly citation totals from each year of the MEDLINE/PubMed Baseline referencing citations back to year 1781. These totals may increase over time for a particular year as new citations are added. For example, 25 citations were listed for the year 1800 in the 2018 MEDLINE/PubMed Baseline, while the 2019 Baseline includes 387 citations for that year.