Write it in Microsoft Word and use
this to convert it to a PDF. Just upload your word document, hit convert and then download to your computer. To make it look professional I would go to Fiveer and get an ebook cover made.
To make the writing of the book easy and fast you could purchase a PLR (Private Label Rights) ebook and spend a little time editing, adding to or re-writing the content and adding a new ebook cover. If you don't have the ability or time to write or re-write content then hiring a freelance writer would be your best bet if you want to create an ebook fast and easy.
Tell them what you want and have them re-write a PLR product or write your ebook content from scratch. Hope my reply helps!