In Preview, is it possible to insert a saved signature into a PDF? I get forms that need to be signed. I could print the forms, sign them, and scan them back into the computer. That's a waste of paper. Sometime ago I scanned my signature into the computer. I saved the signature in three different formats, PDF, PNG, and JPG. I can insert these signatures into Word documents. Can I insert such a signature into a Preview document?
Mac mini, macOS 10.15
Running Big Sur 11.1 on Intel Retina MBA and as per the Help menu within Preview
Create and use signatures
To sign PDFs, you can capture your signature using your trackpad, the built-in camera on your Mac, or your iPhone or iPad.
on your Mac, click the Show Markup Toolbar button 
(if the Markup toolbar isn’t showing), then click the Sign button 
., then click the signature to add it to your PDF.If you use iCloud Drive, your signatures are available on your other Mac computers that have iCloud Drive turned on.
I tried the Control key during the screen shot, and I did get a result. When, as an experiment, I opened a new window in Preview, the new window opened with the content of the screen shot. So the Control key does appear to save the screen shot to the Clipboard.
I have looked in the information window of the PDF into which I want to insert my signature. This PDF is written by someone else. The information window says that I can only read and write on this PDF. I am guessing, then, that read-and-write does not include pasting.
Could that be the explanation?
I want to double-check. The document into which you pasted three times the error message looks itself like a screen shot. Is it? If so, that might explain a lot, because Mac saves screen shots as PNG and not PDF.
I have a Mac Mini. It does not have a camera.
As I said, the Trackpad is useless. When I put my finger on the Trackpad, the Sign tool starts drawing a line dead center in the middle of the Sign window. If I start writing my name there, I very quickly run out of room in the right half of the Sign window. If, without lifting my finger, I move it to the left side of the Sign window, Preview draws a line over to where I want to start writing my name. I do not want an extraneous line above my signature. If I pick up my finger and move to what I would like to be the left margin of the sign window, Preview starts a new line dead center in the middle of the sign window. Then I draw another unwanted line over to the left margin of the Sign window. I sign my first name and lift my finger in order to separate my surname from my first name. When I place my finger back on the trackpad, Preview starts a new line dead center in the middle of the sign window. I can then run another extraneous line down to where I want my surname to start. If I can squeeze my surname into the Sign window, I need to go back and insert the dots on the letters "i." I lift my finger off the trackpad. When I place the finger back on the Trackpad, Preview stats a new line dead center in the middle of the Sign window. I can then draw a third and a fourth extraneous line down to where the dots in the letters "i" are. Instead of a recognizable signature, I end up with something that looks as if it was created by a sot on an Etch-a-Sketch.
As to creating a signature on my iPhone, I looked in my Mac's Preview. It has no "Select Device" tool. I did a search for the word "device" in Preview's help window. Preview reports that it cannot find the word "device" anywhere. FWIW my Preview is Version 11.0 (999.4).
[Posted in error - can't see how to delete it]
It has been a long time since being faced with this issue. Years ago, in Preview the Camera Option would allow it to use built-in camera to take a Photo of you Signature and integrated into the Signature Feature of Preview. Like I said, has been years since last I tried especially since Big Sur. My signature in Preview has followed me from El Capitan all the way to Big Sur even when doing Erase and Clean installs. Guess I have been lucky
Yes. You can just Copy your signature, paste it on the opened PDF file and then drag out where you want it.
Or you can use Preview's Signature feature:
Add signature to PDF?
