What's the point of these two ebook formats? The quality and file size of the ebooks tend to be near identical and ereaders for these aren't exotic at all.
Why would you choose one over the other?
Well in my opinion and also testing knowledge of ereaders... the djvu and pdf doesn't hold that great difference anymore... In the past djvu use to have a higher compression ratio cause the files to be much smaller in storage and also had higher quality rates at this compression... Now with tech as far as now pdf is also able to have high compression ratio... Though when it comes to ereaders you see pdf is sometimes much more secured when doing DRM as such it would be more used above djvu... also I know from tft eReaders they tend to not always be able to support the djvu format that greatly where as all tft eReaders do support pdf, due to the fact that most development with eReaders ensure adobe support so that people can get DRM ebooks. E-ink readers are able to support djvu again very good so yeah on the end nowadays I would say use the format of the ebooks easiest for you to obtain :D
I hope that my explanation help a bit. This is my personal opinion from my experience of testing across different eReader devices and E-ink as the company I work for use to back in the day import them when they were big in the market and I was lead tester for many of these devices :)
Also seeing now the title you wanted advantage and disadvantages... okay well then
pdf: safer, more secured, available across more platforms, but more detail slower loading, refresh ratio is low
djvu: better caching for E-ink, redraw faster, graphical book, not as secured, not available on many different platforms
Philosophical question. Windows vs Linux? MS Word vs LibreOffice? The enlightened part of the population will use Djvu. Others will use the PDF because the existence of DjVu is below their distinctive ability.
How the question should be?
Why is the PDF expanded? - Because Adobe. And Adobe is a big corporation.