How do I fill in a downloaded form?

Bank form that was e-mailed to me. Can it be filled in on Libre Office?

What type of “form” is it - MS Word document, PDF, something else?

it’s a pdf form…but the first question was is it possible?

Maybe; but my suggestion would be to use a more appropriate tool, a PDF reader

Banks are not particularly high-tech, often their forms are intended to be printed out and filled in by pen. In other words, they are often a simple pdf without any form fields, only dotted lines or simple rectangles.

If the form cannot be filled in by a pdf reader then open it with Draw and insert text boxes (Insert > Text box) and type in your information