Don't Use Online Proofreaders or Plagiarism Checkers

Warning.

Do not use online "free" or "free trial" sites that promise to proofread your writing or check for plagiarism. 

Virtually all of them require you to click "Agree."  In the fine print of the agreement, you give them rights to your document.  And they have an affiliate that sells papers to students.

Here is some typical "fine print" from dictionary.com's "proofreader free trial":

By making any material or information available through the Site, you automatically grant to Dictionary.com a worldwide, royalty-free, perpetual, irrevocable, and non-exclusive right and license to use, reproduce, modify, adapt, publish, translate, distribute and sublicense any such material or information (in whole or in part) and/or to incorporate it in other works regardless of form, media, or technology. By making any material or information available through the Site, you also grant to users other than yourself the right and license to access, view, store, or reproduce your material and information for that user’s personal use.