Ajdukiewicz and some other classics, online

If you read German, Ajdukiewicz's stuff on syntactic connectivity is available here. His paper on the notion of existence (in English) is here.

Mostowski's classification of logical systems is available here.

Tarski's paper on the notion of truth is here (in German). His "Semantic conception of truth..." in English is available here.

Reichenbach's Elements of symbolic logic are here.

Principia Mathematica can be found here.

Carnap's Empiricism, Semantics and Ontology is here, and his Meaning and Necessity is here.

Ayer's Language, Truth and Logic is here.

The Need for Abstract Entities (Church) is here.

Austin's plea for excuses is here.

Soles' paper on Russell's causal theory of meaning is located here.

Angelelli's paper on disputation in the history of logic is here.