I noticed that Adobe Acrobat Reader 2017 (2017.011.30156) manipulates an opened PDF immediately, so that it asks to save the changes on exit, even if apparently no changes were applied. I was unable to find any setting which would justify an implicit modification.
I never noticed that with PDFs not having been generated by TeX. My documents are generated using LuaLaTeX and XeLaTeX.
Without specifying PDF version, TeX generates PDFs 1.5 which are converted to 1.6 by Acrobat Reader. I don't know why, but at least I understand what happens. Using the answer at Options controlling PDF Output I changed the PDF version to 1.7, but still the Reader modifies the PDF.
"Diff"ing original and acro-saved PDF, I notice several changes I do not understand, like
At the head of the document, Acrobat introduces a stream object
<</Filter/FlateDecode/First 1047/Length 3801/N 122/Type/ObjStm>>.
At the end of the document, an XMP-META object
<</Length 3171/Subtype/XML/Type/Metadata>>is appended.
<</ColorSpace[/Indexed/DeviceRGB 200<...>]/Type/XObject/Subtype/Image/Width 1493/Height 417/BitsPerComponent 8/Filter/FlateDecode/Length 5670>>is restructured to
<</BitsPerComponent 8/ColorSpace[/Indexed/DeviceRGB 200(...)]/Filter/FlateDecode/Height 417/Length 5670/Subtype/Image/Type/XObject/Width 1493>>.
<</Length 376/Filter/FlateDecode/Type/XObject/Subtype/Form/BBox[0 0 350 98]/Resources 21 0 R>>become restructured to
<</BBox[0 0 350 98]/Filter/FlateDecode/Length 376/Resources 21 0 R/Subtype/Form/Type/XObject>>
Any idea what happens here and how to avoid it? It's annoying!