I always forget how to insert my PNG signature into PDFs using the Preview application on macOS because the straightforward copy-and-paste method doesn’t quite cut it.
The workaround described bellow allows you to bypass this limitation by treating the image as an object that can be copied and pasted.
Steps
- Open the source image file and target PDF using Preview
- Press the following key sequences:
- ⌘+A(select the entire image)
- ⌘+C(copy it to the clipboard)
- ⌘+V(paste the image on top of the original as an- object image)
 
- Click on the object image you just pasted on top of the original image
- Press ⌘+Cto copy the new object image to the clipboard
- Click on to target page and press ⌘+Vto paste the image into it
Remember, this process essentially involves copying the image twice: first from its source file and then as an object image from within Preview. This converts the image into a format that can be seamlessly integrated into your PDF document.
Source: https://www.geekytidbits.com/how-to-insert-image-on-pdf-with-apple-preview/