I have a document, which I am more than happy to share with you, for which I get the following warning both in Chromium 57.0.2987.98 on Ubuntu 16.04 (64-bit) and Firefox 52.0.2 (64-bit):
Could not save annotations this type of PDF file. This should not have happened. Please contact email@example.com
The "annotations this type of PDF file" bit, which should probably read "annotations for this type of PDF file", is not my mistake, I copy it verbatim.