Similarities :
PDF and PDF/A-1b seem to be pixel-perfect matches, according to the tests we have been running.
Size is about the same, but of course this depends on font embedding.
Points of interest for PDF/A-1b :
The content of PDF/A-1b needs to be valid (for example : no images with transparency can be used)
Fonts need to be embedded
No color profiles can be used
Encryption is not supported, so password security is not possible