Ask HN: The P in pdf is a lie?

Yesterday I received a form from a third party in .docx format. They probably use MS Windows and MS word to create it.

I am on a mac. So I opened the file in Pages.app and export it to pdf. I then proceed to complete the form with Preview.app. After that i saved my pdf and emailed it back to third party.

Today third party replied: the pdf file is corrupted. Can you please send .docx file?

Did I do something very stupid or indeed Portable Document Format is a big lie?