Open RPTDEF File
RPTDEF itself is not an extension, but it is a DLL file used by the Crystal Enterprise Report Application Server software.
The RPTDEF files contain a definition of the format of the reports issued by Crystal Report.
DLL files are types of system files, more specifically known as the Dynamic Link Library format, hence the acronym DLL.
Any DLL, including the RPTDEF, is subject to replacement, in case of damage or loss of the file. On the internet there are vast databases of DLL files that allow them to be downloaded for later replacement.
Normally DLL files, even the RPTDEF, are clear targets of computer hackers, when developing malicious software, therefore, if you need to replace the RPTDEF file, you can find an updated version of said file, for example, in the following database: