I was wondering what peoples opinions are on what compatibility level you set your PDFs. The lower you decided the higher the file size, so I assume it's a balance between optimizing the file size and supporting older adobe versions. Does anyone know the percentages of users use below version 8?
Reference https://acrobatusers.com/tutorials/understanding-pdf-compatibility-levels
PDF has been an ISO standard (ISO 32000 part 1) since 2008 which corresponds to your PDF 1.7 entry. We now have 2015.
Thus, the default assumption should be that it is safe to use PDF-1.7.
The page you link to is from 2009. Back then those compatibility levels indeed were a concern.
PDF 2.0 is in the making as part 2 of ISO 32000. As soon as it will be published, we might have to think about compatibility once again.