I want to understand you clearly. When you made the scan, did you specify .rtf? If so, you can open that in Word. After editing, save as .doc or .docx. Then convert it to PDF.
I didn't know you wanted to write on it. Now you're forcing me to remember how to do that kind of thing in Word
One thing you can do is type your text on a page, then overlay the image on the text. In the properties, set the image to be behind the text.
Another way to anotate in Word is to select Insert\Shapes. Pick one. Place it on your picture, right click and select Add Text. You can right click and select the desired font, etc.
Note you can do that in Word with any inserted Picture. So, if you scanned to .jpg, all you would have to do is insert it.
I'm sure there are other ways but I don't have time to investigate.
I'm not sure what you're wanting to do. If you scanned the image as a .jpg or similar, you could annotate it in something like Paint. Maybe you can tell us what type of material you're scanning and what you really want to do with it.