Is compressing data prior to encryption necessary to reduce plaintext redundancy? As explained in William Stallings' Book, in PGP encryption is done after compression, since it reduces redundancy.

I couldn't relate encryption strength with redundancy. Could anyone explain more on that?
There is at least one way in which compression can weaken security; it has to do with the fact that essentially all methods of encrypting arbitrarily long message will inevitably leak information about the length of the input.  The only way to avoid this leak is to pad all messages to a constant length before encrypting them &mdash; but if the messages are compressed, the compression will change their length in a way that will depend on their content, which may allow an attacker to obtain information about the content based on the length of the ciphertext.

For a practical example of such an attack, see e.g. "Uncovering Spoken Phrases in Encrypted Voice over IP Conversations" by Wright et al. (2010).