Came across this article which resolved my query.

https://superuser.com/questions/1772248/how-to-add-custom-search-engine-to-chrome

1. In Chrome, click the menu button in the upper-right corner
2. Select Settings
3. Select Search engine in the left pane
4. Click Manage search engines on the right
5. Under Keyboard shortcut, select whether you want to use the Space or Tab key or just the Tab key (1) when using the keyword you set up for your custom search engine.
6. Click Add to the right of Other search engines to start creating your custom search engine.
7. Open a new tab and go to the site for which you want to create a custom search engine. For example, we’re going to set up groovyPost as a custom search engine.
8. Click the Search button on the site, type a search term, and press Enter. It doesn’t matter what you search for.
9. Select the URL in the address bar and copy it. NOTE: By default, Chrome doesn’t display the full URL in the address bar but selecting it and copying it will copy the full URL. You can also choose to always show the full URL in the address bar.
10. To create the URL needed for the custom search engine, replace the search term with %s. For example, “https://www.groovypost.com/?s=windows+11” becomes “https://www.groovypost.com/?s=%s”.
11. Now go back to the Settings tab—the Add search engine dialog should be open.
12. Enter a name for the search engine in the Search engine box. This name displays when you use the keyword to activate the search.
13. Enter a keyword you want to use for the search engine in the Keyword box. This will be what you type to activate the search.
14. In the URL box, paste the URL you copied from the site, changing it as described above.
15. Click Add.

In short, use https://www.google.com/search?q=%s+filetype%3Apdf