While people on the Internet are still fighting about npm vs bower, there are some packages that are only available for npm and some that are only available for bower. Unfortunately you will run into problems, sooner or later, just like I did today.
But if you want to use npm and want to avoid bower (for whatever reason), then you will run into a problem.
Thankfully, somebody created a wrapper package for npm: https://github.com/AaronBuxbaum/pdfmake-client
You can install it via
npm install pdfmake-client --save