Use older version of pdfjs-dist as newer version breaks pdfTree

[larryhudson]

Using the latest pdfjs-dist version (as of March 2021) breaks pdfassembler - I got the following errors:

• when using getPDFStructure, ['/Root'] would be undefined
• when using resolveNodeRefs, this.pdfManager would be null

Using the older version of pdfjs-dist fixed the issue for me.