A pdf is an initial stage in the making of a photobook. It can be a rough beginning that can provide something to assess as a dummy book before going to a publisher or printer.
A pdf entitled This is Not a Book is currently in progress.
There is a completed pdf provisionally entitled Walking Wellington. It was initially started as a Tumblr blog entitled Reconnections circa 2020. It was envisioned as a photobook with text on the left page and the image on the facing right page. The pdf will be uploaded in the near future.