Please suggest some good free software to create photo books, which can save in PDF format.
If your needs are fairly simple google docs (free) will allow you to create presentations (better interface for a book, but the aspect ratio is fixed) or documents (more like a word processor, but you can change your page size) which can be saved as pdfs (free). You can also find some (very rough) templates online.
There are good choices for free software, but not free books — for example AdoramaPix has a pretty good web interface, good quality books and optional (not free) pdf creation.
Both Lightroom and Aperture (not free) have book modules that will generate pdfs (for free). Having one of these two programs is worth it, if you are at all into photography and they both do a nice job.
The big question is what are your requirements?
Just some photos in a pdf? Word (careful, compressed images by default), LaTeX, Libre Office... Proper "design"? Then you want something else.
One of the "go to" tools is InDesign - however I have not used it and it is expensive. Lightroom4 (and newer possibly too) can do photo books - but I'm not sure in what format they would be saved.
The key question to ask yourself is, what do you expect from your tool? Just "some photos" per page? Edge to edge prints including full layout and colour management control?
Lastly: If you are thinking about a printing service such as Blurb or Whitewall Germany, then it might be best to use their simple but free tools - even if that does not give you a pdf but just the finished product (true for blurb) - before you consider getting some more sophisticated tools.