You can take the first 100 pages and copy them to a new document and run the spell checker.

Fix any actual spelling mistakes. If something is mistakenly identified as a spelling error because it is in a different language, change the proofing language for that word or phrase to the correct language.

If it is a correctly spelled word that you want always recognized as correct, add it to the dictionary.

If it is a correctly spelled word that you do not want to add to the dictionary (i.e., a proper noun) mark it's proofing language to "do not check spelling or grammar."

You can handle marking the proofing language for words and phrases using character styles.

One you have finished that 100 pages, copy it and paste it back into your big document, replacing the original text. Go to the next 100 pages.

Mastering the Spelling Checker