I used the Book Module in Lightroom Classic to prepare a photobook. The book has several double-page spreads with one photo spreading over 2 pages. In LR the layout looks perfect, but in the pdf it comes out wrong.
The reason is that in LR page 1 is on the right, while in the pdf page 1 is on the left. Thus, if - in Lightroom - a double-page photo is on p2 (left) and 3 (right) it looks as intended. However, in the pdf the photo will be on p2 (right) and p3 (left) and be cut in 2.
How can I prevent this in the pdf?
Since LR was designed to work with Blurb, will Blurb print the correct LR version or the incorrect pdf version?
You cannot "prevent this in the pdf.". Blurb (if you send a PDF) will 'see' each individual pdf 'page' and print them starting at Page 1 which will be on the Right (the first printable page in a book). So it will print as intended, double-page spreads included.
Anyway, for a Blurb book, the only reason to produce a PDF is to check the content of all pages. When you have chosen a 'Blurb Photo Book' in the Book Settings panel, a click on the button [Send to Blurb] will handle the pages as you designed. No need to 'save' or send a PDF.
If, for you own purposes, you want images of the book that keeps double-page spreads as one singular image, then you would need to set 'Book Settings' to "JPG" and then Export JPG files.