Apple - "The document could not be saved" - Preview error

I was facing the same issue, I finally got the issue resolved by following steps:

  1. Go to File
  2. Click option 'Export as pdf'
  3. Save the file name with pdf extension (like 'file_name.pdf')

This worked for me, so give it a try.

I realised this is a bug in the Mask tool. Remove the mask and the image saves without any problem. Avoid this tool until Apple addresses the issue. Checked on High Sierra 10.13.4.

The Mask tool in Preview

A solution to this is to take a screenshot of the image preview before saving it(solution from @gpbl).