Sometimes a PDF file or Office document fails to convert. The reason for this can be varied; in the case of PDF files it may be that the file has security restrictions, printing disabled or a password in place. In those cases the conversion can fail, resulting in a slightly in-descriptive 'Error: upload failed' error.
Sometimes it is possible to re-save the file without the restrictions in place.
When it comes to Office file formats such as DOCX then it may help to save as DOC or PDF.