Improve UI when inserting PDF from URL

[snazare]

Is your feature request related to a problem? Please describe or add steps to reproduce your use case.

Currently when you try to use an URL in the PDF macro modal, you need to paste it in the File field, hit "Enter", then Submit. If you paste and Submit directly, the field gets a red highlight and prompts the user to upload the file.

Describe the solution you'd like

Ideally, the user should be able to paste the URL, then hit submit.

Describe alternatives you've considered

Alternatively, we can make a dedicated field for PDFs from URLs