You said "by default" which is correct but just for the sake of completeness, the built in Chrome PDF reader can easily be added to Chromium on Arch using this package from the AUR: https://aur.archlinux.org/packages.php?ID=44148

I've been using it for almost a year now without issue.

Me too! In fact, that's why I said "by default" :)

Similarly, Pepper Flash Player can also be integrated from AUR.

There is also a Google Chrome/Chromium extension also provided by the pdf.js team. (It's not in the webstore, though.)

I've used it since about half a year and I like it a lot. (Not having to download PDFs all the time and having PDFs in tabs alongside the sites I read.)

I agree. Separate windows for PDFs is terrible.

You can also separately download the proprietary PDF reader of Chrome and use it with Chromium. If you are using Arch Linux, it is available in the repositories.