AEM Forms designer is used for designing XDP (templates) which are used in AEM Forms components. XDP defines the placement of elements like fields, text areas, etc. This template is eventually merged with data to create the form. The form can be a PDF or HTML form (in AEM Forms). I hope clarifies your doubt.
You can't download it normally. If you need a trial license, contact your local Adobe sales rep and they can generate a Trial license download email of the Adobe AEM Forms Designer software for you. The software is included with any AEM Forms license.