These PDFs apparently used the “incremental update” feature of PDF, where edits to the document are merely appended to the original file.
It’s easy to extract the earlier versions, for example with a plain text editor. Just search for lines starting with “%%EOF”, and truncate the file after that line. Voila, the resulting file is the respective earlier PDF version.
(One exception is the first %%EOF in a so-called linearized PDF, which marks a pseudo-revision that is only there for technical reasons and isn’t a valid PDF file by itself.)