Is it possible to decrypt cryptowall-encrypted files?
Cryptowall (from about version 3 and onward) uses AES encryption. The kind of attack you are describing to recover the key(s) used is a known-plaintext attack. Even with an absurd amount of plaintext-ciphertext pairs, there are no known reasonable attacks on AES in the foreseeable future. The AES key itself is encrypted with RSA, but the known plaintexts aren't relevant, as the RSA key is not used to encrypt your files directly.

If you can't live without the documents, your only real option is to pay the ransom. The only other option is to live without your files, and hope that in the future the perpetrator is arrested and the keys are released.

And of course, be sure to keep your antivirus software up-to-date and follow other good practices in the future.