https://convertio.co/ offer free file conversion service with following limitations:
- Max file size: up to 100 MB each
- Max number of conversions in the last 24 hours: 10 files per 24 hours
- Max number of concurrently launched conversions: 2
There are no other restrictions. If you exceed any of these limitations you will see an offer to purchase one of our premium plans for file conversions.
Please, also, note that we have different free tier limits for text recognition (OCR).