How to initialize Assets in Head view in widget?

Question

Yii

I want to embed Pdfjs into yii2 as a widget, but I don't remember how to initialize css and js files in views/viewer.php . How to do it?
Link to GitHub: https://github.com/sofvlad/yii2-pdfjs
I will also write it, because it seems there is nothing complicated, but I can’t even do it because I had no experience in writing widgets and extensions.
PS The widget is written for the advanced version of yii2 in common/widgets

Answer 1

Good evening.
Read and reread .
Everything is painted to the smallest detail.
Also all other sections of the documentation - read and re-read.