I saw this here.

Problem:
A comma is missing, or the adjective “incredible” is inappropriate here.

Explanation:
“Foxit Reader is a … viewer …, with incredible small size …” contains two adjectives in a row.

Either the writer wanted each adjective to modify the noun “size”, or the writer wanted to modify the second adjective to modify the first adjective.

If the writer wanted “incredible” and “small” to modify “size”, then a comma should go immediately after the first adjective — yielding the first solution.

If the writer wanted to modify “small”, then a different solution is required.

When one wants to modify adjective Y with adjective X, one must convert adjective X into an adverb.

Converting adjectives to adverbs usually requires adding “ly” and sometimes requires dropping a letter or two.

Converting the adjective “incredible” to an adverb yields “incredibly” — and the second solution.

Solutions:
“Foxit Reader is a … viewer …, with incredible, small size …”
or
“Foxit Reader is a … viewer …, with incredibly small size …”