This is pretty much 100% a note to self, but if somebody else finds it useful, great.
If you submit a PDF to the patent office, it seems that 10 times out of 9 you get some error, and this is the most worst of them all:
The attached PDF file references a non-embedded font: Arial. Please remove file, embed the font and reattach.
The requirement is to use to convert your PDF into a PDF-A format, short for PDF Archive, that includes all of the fonts embedded into the document.
If you are generating a PDF from a Word document, there is an option to specify that the document should be generated as a PDF-A.
In Word: File > Export > Options > check the PDF/A checkbox > OK > Publish
Ironically, this option of printing to PDF-A is not available if you are using Acrobat Reader. Even if you use an application like Solid PDF Creator, you will still get the error.
The key is to upload an updated and saved version of the ]editable form that you downloaded from the PTO website. Do not print it as a PDF.
So, in summary, for Word, use the File Export option referenced above, and for Acrobat Reader just upload a saved version of the editable form.