Let us say I want the .pdf of the railway signalling book "Locking: Being an Elementary Treatise on the Mechanisms in Interlocking ... (1907)" which can be found on archive.org at http://archive.org/details/lockingbeingane00lavagoog
On that page I am offered various formats, .pdf, .epub, Kindle and Daisy, in addition to reading online.
But if I click on the .pdf line, I am sent to a google-books page where I am told "No ebook available".
When confronted with such a lie, do not loose hope; go back to the archive.org page, copy the link location of the Epub or Kindle format, paste it in your browser and, before you press Enter, replace the final .epub or .mobi with .pdf
And surprise ! The pdf file starts downloading, in spite of not existing according to Google...
It seems this applies to all the books in archive.org that have been digitized by Google.