Is there any way to download the PDF file from CMS right after clicking the form Submission button.?
Is the form on a CMS page? If so, you could put the download link to the form in the Success state of the form.
If it is not, you could do something similar with a CMS Collection List within the Success state. However, if you wanted it to dynamically show a different PDF out of multiple CMS items, that might not work.
Go to the form settings in the Designer
Change the state to Success
In the Success state Div Block that is now visible, add the link or CMS Collection List
Hope that helps.