This depends on what you want to do with the document afterwards.
If you intend on downloading the document directly, and only downloading it, you can upload PDFs up to 15 megabytes.
If you want to have the document emailed to your participant, then the limit is 7.5mb.
Why is there a limitation at all?
When you bring a document into a SuiteBox meeting, we must convert it in order to allow for signature placement. And then finally, when the signature is placed, we must convert it again.
This conversion process can increase the size of the document by about 25% (sometimes it can shrink it!).
We cannot host files larger than 20mb available for download, and we cannot attach files larger than 10mb for email delivery. We intend to improve this significantly in the months to come.
In the meantime, you may want to consider a PDF optimization solution such as NitroPDF, which can shrink your PDFs to a smaller initial filesize.