After trying so many different solutions for this issue now I've found that both the addHTML and fromHTML is depreciated in latest versions of jspdf and they are replaced with just html. For those who are seeking for a better solution to work with jspdf and Angular 10 here is what looks working and a better result that I've found so far.

To import jspdf version (2.x.x), just simply do: import { jsPDF } from 'jspdf';.

You won't need to include any of the dependencies into angular.json scripts like you would do in previous versions. And if you add scripts of jspdf or html2canvas insideindex.html, also remove them. By running npm install jspdf it also install the html2canvas which is also dynamically imported by jspdf.

import { Component, OnInit, ViewChild, ElementRef } from '@angular/core';
import { jsPDF } from 'jspdf';

.....

 @ViewChild('couponPage', { static: true }) couponPage: ElementRef;

.......

openPDF(): void {
  const DATA = this.couponPage.nativeElement;
  const doc: jsPDF = new jsPDF("p", "mm", "a4");
  doc.html(DATA, {
     callback: (doc) => {
       doc.output("dataurlnewwindow");
     }
  });
}

Though for me still using doc.html() method, it wouldn't use the css style that I used for my page.

I have done it, after doing lot of R&D , their are few steps to follow as below : Install :

npm install jspdf --save

typings install dt~jspdf --global --save

npm install @types/jspdf --save

Add following in angular-cli.json:

"scripts": [ "../node_modules/jspdf/dist/jspdf.min.js" ]

html:

<button (click)="download()">download </button>

component ts:

import { Component, OnInit, Inject } from '@angular/core';
import * as jsPDF from 'jspdf'
@Component({
  ...
  providers: [
    { provide: 'Window',  useValue: window }
  ]
})
export class GenratePdfComponent implements OnInit {

  constructor(
    @Inject('Window') private window: Window,
    ) { }

  download() {

        var doc = new jsPDF();
        doc.text(20, 20, 'Hello world!');
        doc.text(20, 30, 'This is client-side Javascript, pumping out a PDF.');
        doc.addPage();
        doc.text(20, 20, 'Do you like that?');

        // Save the PDF
        doc.save('Test.pdf');
    }
}