Every time you handle an original document, you contribute to its destruction.
If there is a copy available, work from that if you can. If it leaves you with an unresolved query which sight of the original might resolve, then ask to see the original.
Whether having sight of the original resolves the query or not, consider asking if you can lodge a note with the copy for the benefit of future researchers, saying "Having viewed this copy, I was left with the following uncertainty. I therefore viewed the original, with the following outcome: [here record your observations and conclusions]. Sign and date it.
Future generations of researchers will be grateful for any efforts you take to prolong the life of the documents in which you and they have a common interest, but in respect of which you have the unfair advantage of being allowed to take the first look ...