On OS X Mavericks 10.9 and earlier, enter Preview Signature Privacy in the Keychain Item Name field.Click on the + icon to create a new entry.Open Keychain Access as above and do the following: This is necessary when the target machine (like a Mac Pro with an older or third-party display attached) has no camera which means no Signature file can be created from within Preview. In situations where the target machine has no existing Preview Signature Privacy or Signature Annotation Privacy record in Keychain Access, you must create a new record. Click the Show password checkbox and paste the password you copied from your original machine.On OS X Yosemite 10.10 and later, locate the Signature Annotation Privacy password in the login keychain and double-click to edit.On OS X Mavericks 10.9 and earlier, locate the Preview Signature Privacy password in the login keychain and double-click to edit.On OS X Yosemite 10.10 and later, copy the file to the folder, overwriting any existing file.On OS X Mavericks 10.9 and earlier, copy the .plist file to the folder, overwriting any existing file.Launch Preview, open Preferences, and select the Signatures tab to ensure the default configuration files and folders have been created.You'll need to transfer this to your other computer(s). Paste it into a text editor and save the file.On OS X Yosemite 10.10 and later, right-click the Signature Annotation Privacy password and select Copy Password to Clipboard.

This is the password used to encrypt the signature images.

On OS X Mavericks 10.9 and earlier, right-click the Preview Signature Privacy password and select Copy Password to Clipboard.

Ensure the login keychain is selected and choose the Passwords category.

On OS X Yosemite 10.10 and later, copy the file.

On OS X Mavericks 10.9 and earlier, copy the .plist file.

Alternatively, press ⇧⌘G whilst Finder is active and enter the path above to directly navigate.

In Finder, click the Go menu and hold option to show the Library folder.

Open the ~/Library/Containers//Data/Library/Preferences folder.

Here are the steps to get both the file where the signature is stored as well as the associated keychain entry to a second Mac.

This was covered by Aussie Bloke's blog when Lion arrived.

How to encrypt pdf in preview mac os x