There are character limits on various object names that may or may not differ between versions of Splunk. For example, in 6.3 the name of a report is limited to 100 characters maximum. Thus I imagine what you are experiencing is a character limitation within the software for the file name of the attached PDF file sent within the e-mail. In your case, 62 characters are consumed before the .pdf extension with 11 characters consumed by the date (-2016-02-09).
All this said, I am checking to see if there is a published set of name character limits for Splunk.