Project Blue Book was the code name for United States Air Force's study of unidentified flying objects (UFOs -- now often called UAPs), which ran from 1947-1969. Over 12,000 reports were collected before the project was terminated. Project Sign and Grudge were the fore-runners of this effort and are also included in this collection.
After extensive expert review, the Air Force claimed that the vast majority of the incidents could be accounted for by conventional explanations -- with about 700 incidents still unexplained. To this day, there is active debate surrounding the accuracy and transparency of this conclusion.
It's complicated.
They were declassified in 1975 when the US Air Force transferred the UFO files to the National Archives; however it took over a year to redact all the personally identifiable information from them and make them available to the public in 1976. For decades the records were only viewable on-site in the National Archives Building in College Park, Maryland. In paper format, they take up about 42 cubic feet. The full collection is estimated to be over 120,000 pages.
The story of how the documents were digitized is also eventful and shrouded in controversy. The effort to make the documents digitally available dates to 2004 in which software developer Rebecca Wise OCRed about 1/3 of the pages. This stood as the sole digitized compilation of Blue Book files until 2007 when the Fold3 website published digital scans of around 130,000 pages (some of which were duplicates).
Then legal conflict over the online posting of Project Blue Book files emerged in early 2015 when John Greenewald published the documents in pdf format (making them easier to download as opposed to the jpgs on Fold3) on his website Black Vault. Soon after Ancestry.com, the parent company of Fold3 (formerly Footnote.com), issued a takedown demand to Greenewald, who had spent decades filing FOIA requests to make government UFO records publicly accessible. They threatened Greenewald with legal action if he did not remove the collection, claiming that Fold3 held the “copyright” to the digitized version of the files. Greenewald has since removed and then reinstated Black Vault, however the site does not contained OCRed versions of any of the documents.
For more information see pages 6-9 on Brad Sparks 2020 work Blue Book Unknowns.
The files come from 12,000 reports of UFO citings between 1947-1969. The pages fall into a few main categories: memos, forms, tables, administrative files, maps, and newspaper clippings.