pdf printing different content from the on-screen content

I sent a pdf of a pages document to another organisation just now. It looked fine on screen and it was fine when received but when printed it came out as a combination of the document I sent plus other text. I asked what was on the text and it was clearly another document of mine and could not have been on the printer already or on the recipient's computer. She could only have received the concealed document from me. I tried printing the pdf my end and it prints fine. So how did this happen?

I am using pages 5.6.1 and preview 8.1 running on a macbook pro mid 2013. I can only surmise that the content was in the pages and pdf document somehow from a a previous cut and paste but not visible. Clearly I am anxious that this should not happen again even though this time no harm was done. I have gone back to the pages document and deleted the content that is meant to be there. The rest is a blank page. Grateful for advice.