Learn Farsi: بزرگ‌ترین محققان ایرانی این سندهای مهم را ترجمه کرده بودند. - The greatest Iranian researchers had translated these important documents.

Because 'researcher' (محقق) is a living person. In formal or written Farsi, animate nouns (people and sometimes animals) take the plural suffix -ân. You would still hear -hâ used for people in casual spoken Farsi, but -ân is the standard for formal writing.

Why doesn't 'documents' (سندهای) also use the -ân ending?

The suffix -ân is strictly for animate nouns (living things). Because a document (سند) is an inanimate object, it cannot use this formal ending and must take the standard plural suffix -hâ.

How do we connect محققان (researchers) to the adjective ایرانی (Iranian)?

You just use a standard ezafe (an 'e' sound). Even though محققان has a special plural ending, it still acts like a normal noun when adding adjectives. It is pronounced 'mohaghegh-ân-e irâni'.

Why does بزرگ‌ترین (greatest) come before the noun, while ایرانی (Iranian) comes after?

Superlative adjectives (the 'most' or '-est' forms ending in -tarin) always go before the noun they modify, and they do not need an ezafe. Normal adjectives like ایرانی go after the noun and must be connected with an ezafe.