Not all PDFs are created equal

I was not too happy about a PDF book "Force ..." not able to open be opened in GoodReader (on iPad).  But then I found out that the same PDF file also couldn't be opened in iBook.  I have no problem reading the PDF file on MacOS, so something must be causing the PDF failed to work on  iPad.  No big deal I guess.  This is more like an expectational case than a norm.