You can upload PDF, PPT, PPTX, PPTM, PPS, DOC, DOCX, DOCM, Pages, Key, XLS, XLSM, XLSX, Numbers, TXT, and EPS files.
If you have issues during the upload and conversion process, check that the file name is simple, without special characters or unnecessary spaces.
Also, the byte size of the file is limited by the current plan you have for your account.
Free Plans can upload up to 25mb single files.
PRO Plans and higher can upload up to 250mb single files.