Why do Adobe Reader and Adobe Acrobat ask me to save changes after I open a PDF?

There are two main reasons Adobe Reader and Adobe Acrobat will prompt you to save changes to a file after opening it.

1. The PDF was in some way corrupt and Adobe Reader / Adobe Acrobat automatically repaired it without showing you a warning message. As a result of the automatic repair, the Save button is enabled.
2. The PDF contains form fields and the NeedAppearances entry in the interactive form dictionary is set to true. This means that the conforming PDF reader will generate an appearance stream where necessary for form fields in the PDF and as a result the Save button is enabled. If the NeedAppearances entry is set to false then the conforming PDF reader should not generate any new appearance streams. More information about appearance streams in PDF files and how to control them with Foxit Quick PDF Library.

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