The only entity of your document is the one you uploaded yourself. As soon as you delete a document, no plagiarism check can use it for comparison. The only possibility to plagiarize yourself using PlagScan is, if you upload a document (or slightly modified versions of it) several times and set your data policy to check against your own documents.
Scientifically/technically speaking, if you have published text before, you also need to cite your own work properly in a new publication. It doesn't really make a difference, whether it is your own or someone else's work. Ethically, it makes all the difference, of course.