- [determiner] Whatever you say is fine with me.
- [pronoun] I'll do whatever I can.
It seems like the two ‘whatever’s are doing the same function as pronouns. Why does the Wiktionary put the first one into determiner category?
Wiktionary has two entries under Determiner:
The first of these undoubtedly stands in an adjectival or determinative relation to choice, which is the head of its "free relative" clause. "Relative determiner" appears to be a common term for words of this class.
But I agree with you that the second, the one you cite, acts as a pronoun, indistinguishable from the use in the citation under Pronoun.
Why does Wiktionary call it a determiner?
Wiktionary is crowd-composed, and it looks to me like somebody confused two different uses of the word which both head free relative clauses. The "talk" at that entry indicates that there was some shuffling around involving the Pronoun entry.
The confusion may also be due in part to the fact that what to call the wh- words which appear in this position is up in the air. McCawley, for instance, insists that they are "interrogatives"!