Theoretically, you should get the same quality scan, no matter whether you scan a certificate full scale at 150 ppi or reduce it 50% during scanning and save it at 300 ppi. The math says:.

150 ppi x 10 inches wide x 100% = 1,500 pixels across
vs
300 ppi x 10 inches wide x 50% = 1,500 pixels across

But, the truth is, you DO NOT get the same quality. I have found that it is always better to scan full scale and then reduce afterwards. I advise not depending on your scanning software for image reductions. Here are examples comparing the two approaches.

Scanned at 150 ppi with no reduction	Scanned at 300 ppi with 50% reduction

Why is that?

I don't know for sure. I suspect unpublished assumptions built in the software might explain the universal image deterioration. I have encountered the same results, no matter whether it was a $150 scanner with entry-level software or a $1000 scanner with professional characteristics.

My advice

Scan certificates at full scale and reduce later.

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