With Adobe Acrobat Pro you can try the redaction tool.
We need more information about how that "x" ended up in your document. Here is a (incomplete) list of ways to add content to a PDF document, and every single one of these methods adds a different type of content, which in turn requires a different method to remove it again:
- static PDF content
- form field of type text
- form field of type checkbox
- form field of type radio button
- text annotation
- text box annotation
- stamp annotation
- generic PDF layer
- vector drawing
Unless you can tell us how that 'x' ended up on your scanned document, we cannot help you.