When converting to PDF in the Microsoft 365 version, there are limits on how many conversions you can perform, due to cost considerations. At present, you are allowed up to 1500 conversion credits per list per day. A conversion credit is equal to up to 500Kb of output produced. Any conversion will consume at least one credit. Once credits are exhausted, no more conversions can be performed for the same day and error will be issued.

Note that having embedded PDF attachments is #1 cause of excessive usage of conversion credits. Consider removing the embedding option, removing heavy attachments or ensuring those PDF documents are text and not image based, to ensure the smallest possible file size. 

The limit does not apply on premises or any printing that does not result in PDF output.