A relative pronoun followed by a transitive verb like “find”

1. I've used this type of syllabus which I don't find suitable.
2. *I've used this type of syllabus which I don't find it suitable. (ungrammatical)

This sentence has a relative clause which modifies the noun syllabus. The relative clause is this section here:

• which I don't find suitable.

We understand which here as referring to the same thing as syllabus. If we take the rest of the clause after which as a normal sentence it will seem a bit odd:

• I don't find suitable.

This is because there is a gap there where we would expect the object of the verb find. It must be empty in the full sentence. This gap tells us where the which fits into the clause. In other words we understand the gap as referring to the same thing as which and syllabus:

• I've used this type of syllabus I don't find [ the type of syllabus.] suitable.

We can model the sentence like this:

• I've used this type of syllabus₁ which₁ I don't find ___ ₁ suitable.

In standard English that gap is important for showing where the antecedent noun and pronoun fit into the grammar of the clause. If we fill this gap with a pronoun it can make some sentences ambiguous as well as difficult to understand. In the Original Poster's particular sentence, although it's ungrammatical, it's understandable. This is because there is only one potential space for this gap - even though it has been filled with it (the word which cannot refer to I, because I is a person not a thing or a type of syllabus). However, when there is more than one potential gap, filling the actual gap with a pronoun can cause serious problems

• You know Federer? *Well that man over there is the man I wanted him to beat him. (ungrammatical)

In the sentence above the speaker has filled the gap with the resumptive pronoun him. The problem here is that the gap could be in the place of either the first him, or the second. So the sentence might mean either of the following:

• That man over there is the man I wanted Federer to beat _____ .
• that man over there is the man I wanted _____ to beat Federer.

If we remove the ungrammatical resumptive pronoun, the meaning will be clear:

• You know Federer? Well that man over there is the man I wanted to beat him.

• You know Federer? Well that man over there is the man I wanted him to beat.

The Original Poster asks about whether there should be a resumptive pronoun it filling this gap, because the verb find is usually described as transitive. The answer, for standard Englishes, is No! The verb find actually is already transitive in this sentence. Depending on how you analyse the grammar, the word which or that gap in the sentence is functioning as the direct object of the verb find. We shouldn't insert an extra object here:

• *I've used this type of syllabus which I don't find it suitable. (ungrammatical)