Can I download a form that I already have

I'm assuming what you're hoping to download is your submission data (not your forms). If so, you can export these to XLS, CSV or PDF format.

Complete guide: How-to-Download-Form-Submissions-as-Excel-CSV-PDF

Otherwise, if you indeed wanted to download your form, I'm not sure in which file format you need it to be. Forms are supposed to be filled out online, using a browser. If you need this to be downloaded locally, then a fillable PDF form might suit your needs.

Complete guide: How-to-Create-a-PDF-Form