Thank you for this info @ shubham shiroya

I added the field and modified the form view.
When I checked the table in the database, I can see that the new field image_url has been successfully created there.

This is the updated form view:
<record id="vendor_bill_form" model="ir.ui.view">
<field name="name">vendor_bill_form</field>
<field name="model">vendor.bill</field>
<field name="type">form</field>
<field name="arch" type="xml">
<form>
<group>
<field name="image_url" widget="image" options="{'preview_image':
'https://www.usnews.com/media/best-states/overall-rankings-2021.pdf'}"/>
</group>
</form>
</field>
</record>

And this is my new added field in my model:
image_url = fields.Char(string="Image URL")

Just for an example, I inputted that external link shown above.
But when I update the module, that URL is not being displayed. What can I do to fix it?

I also tried to apply the options attribute to the pdf widget, instead of image widget:
<group>
<field name="file" class="pdf_viewer" widget="pdf_viewer" options="{'preview_image':
'https://www.google.com/images/branding/googlelogo/1x/googlelogo_color_272x92dp.png'}" />
/group>

In this case, when I update the module, if I don't have an image uploaded already, then the pdf file states as missing PDF file:
PDF.js v2.2.228 (build: d7afb74a)
Message: Missing PDF "http://localhost:8069/web/content?model=vendor.bill&field=https%3A%2F%2Fwww.google.com%2Fimages%2Fbranding%2Fgooglelogo%2F1x%2Fgooglelogo_color_272x92dp.png&id=3".