mPDF >=4.3 produces PDF files which are specified as version %PDF-1.4.
1.4 is the lowest specification which contains all the features used in mPDF - with a few minor exceptions:
- SetDisplayPreferences($preferences) allows the option to include "NoPrintScaling" (PDF-1.6)
- Using mirrorMargins, mPDF will set by default /Duplex /DuplexFlipLongEdge (PDF-1.7)
- The "Subject" field of Annotations (PDF-1.5)
mPDF <2.0 produced PDF files which were specified as version 1.3 (the file starts with %PDF-1.3).
mPDF >=2.0 was changed to specify %PDF-1.5, to allow for changes needed using Annotations.