Illustrator created PDFs are not comparable in Acrobat Pro DC?

Hi!
I've created PDFs from a Illustrator file with all possible settings I can imagine of and tried to compare those files in Acrobat Pro DC but I'm only getting a message "Selected document is a scanned PDF and contains no text. Acrobat will perform image to image comparison only." I thought it might be a font issue so I created new a new file with only one circle on it but the message in Acrobat is the same. The PDFs I use most certainly has nothing but vector graphics in them and I'm able to edit the texts and vectors in Acrobat without any problem. Also preflight panel shows no problems in the pdf's. I've also made some changes to the PDF files in Acrobat, saved as a new file and then tried to compare but still the same message. I also tried distilling the pdf from Illustrator but no help. The only way to work around this is that I place the Illustrator created PDF to InDesign and export a pdf from there. Those Pdf's are comparable in Acrobat as are all other pdf's that I've created from InDesign. But as I have tens of packages created in Illustrator I most certainly don't want to go throught such a hassle. And still my client needs to be able to compare the pdf's in Acrobat. I'm going totally crazy trying to solve this.

So what's the difference between the pdf's created from Illustrator / InDesign? How can I save a pdf from Illustrator that will be comparable in Acrobat?