Online processing caps input files at 40 MB - anything larger must go through batch processing with Cloud Storage. If a sync request fails on a file between 20 and 40 MB, you are likely reading old docs or an old client assumption; the limit was raised in June 2025. For files over 40 MB or documents with many pages, switch to the async batch API instead of fighting the sync limit.

Context: Google Document AI release notes (June 19, 2025): the maximum file size for online (synchronous) processing requests was raised from 20 MB to 40 MB across all processor types.