When using GhostScript the version is now usually set to 1.7 so Acrobat 8 and above is recommended / required for viewing. Note Linux & Portable versions stopped with version 9.
The “version” is an indication of the files ability to be read by legacy viewers thus good if it is set at 1.4 (Acrobat 5) or for even more users if 1.3 (Acrobat 4)
The use of PDF/A flags indicates that amongst other requirements it will work with 1.4 compatible viewers and the fonts are included, in a particular fashion but that would not generally apply to CB image files anyway, thus most SumatraPDF image files meet PDF/A-1b – Level B (basic) conformance (except the version 1.7 ones)
Here I have taken PNG scanned pages in a folder and loaded that folder in SumatraPDF to generate a PDF then just for you set Irfanview to force PDF/A compliance NOTE Irfanview will keep the version at 1.4+ even if it was previously 1.7 from GhostScript