Not to necro, but this app is not code-signed. So macOS will not let you open it by default.
User workaround is to open terminal (Use Spotlight (CMND + Space) to find Terminal (App Icon is black rectangle))
Into the terminal window paste the following then press enter:
xattr -r -d com.apple.quarantine "/Applications/Pixel FX Designer.app"
If your App is not installed into /Applications folder, you can paste the following (with a trailing space), then drag the app from Finder into the terminal window then press enter:
xattr -r -d com.apple.quarantine