How may I convert my interesting 28-page manuscript into an e-book for marketing online?

A .pdf document is not secure, the information in it is easy to copy. So if you want to prevent others from copying your book you will need to invest in a program that secures the document so that it cannot be copied

Open Office is a free Microsoft comparable office suite you can use to convert your document to a .pdf file for publishing.

You can save your file as a PDF file using Word OR go to adobe.com and they offer the ability to create a PDF file. You can then offer this ebook/report as a giveaway on your website or as a product to purchase.

If you are using Word 2007 or newer you can save it directly as a PDF from the save as menu.
In Open office you can save it as a PDF from the publish menu.

If you have written it in Microsoft Word, there are plenty of free software programs to convert Word into PDF format.

Use Writer from Open Office.

Go here openoffice.org/download/index.html

Click the green box. On the next page, ignore the form, your download will start automatically after a few seconds.

If someone looks for a specific theme, they search on Amazon. If you convert your book to a Kindle Version and sell it on Amazon, you have a good start and people will find you without you having to do much advertising.

I suggest you do a search on Google for Creating Kindle Books, Kindle Formatting, How to make a kindle book. You will find a lot of free information.