Increase the spacing between paragraphs and bullet points. Basically put, you will want to make sure your page is not filled from top to bottom with text. Make sure that there is white space on your letter. It just makes it easier to scan or read. Limit your paragraphs. This may seem difficult to fit in all of your information, but you will want to keep the maximum amount of paragraphs to 3. Similar to when you read an article on a website, the longer and more paragraphs the content has, makes it seem like a difficult read.
A resignation letter is mostly sent as an email and must be addressed to the reporting manager and the HR. It's not just an information for your boss that you intent to leave but it is also an advance intimation to superiors, and human resources so that they can find a replacement for your position.