There are those who write an ebook and give it away for free because they're intent is not to earn money from their writing
. If the intent is to earn from your ebook download, you always have to make sure that no one is giving away your ebook download for free. It is not a given that the person who is downloading the ebook is dishonest as compared to the one who is actually giving away your ebook download for free.
So, if you want to prevent your ebook download from being distributed for free, you always have to be sure that you include a statement in your ebook pages in the introduction part saying that your ebook may not be given away for free. Another option would be to include a rules doc that among other statements or rules, has this same statement, "May not be given away for free".
Another option would be to have a small portion of the ebook in the introduction part that is available for free and make all the other pages available only after receiving the full payment. You may want to ask the readers to contact you in case they receive the ebook pages for free, asking for full details of from where they received the free version of the ebook download.
In case someone finds a free version, you can always offer them a small reward so that more and more people feel free to help you by providing you with details for the availability of the free versions. You can always take necessary steps to punish people who are making your ebook available for free on the Internet.
These are some of the basic steps that you can take in order to protect your ebook download, especially if it is being done for free. If you are worried that this may not be enough, then try finding some additional methods that others have used with success.