From the article:

pdf-lib was created to address the JavaScript ecosystem's lack of robust support for PDF manipulation (especially for PDF modification).

Two of pdf-lib's distinguishing features are:

1. Supporting modification (editing) of existing documents.

2. Working in all JavaScript environments - not just in Node or the Browser.

There are other good open source JavaScript PDF libraries available. However, most of them can only create documents, they cannot modify existing ones. And many of them only work in particular environments.