If you have a legitimate key but lost the installer, Microsoft still provides official downloads for older versions through their Microsoft.com account portal under "Order history" (requires key verification). Never download from third-party sites claiming to host untouched ISOs – they often inject malware.

Instead, consider these legal options:

Feature-rich with a familiar ribbon interface similar to Office 2013.
Includes: Writer, Spreadsheets, Presentation, PDF toolkit
Note: Free version shows ads but is fully functional.

The best desktop alternative to Microsoft Office.
Includes: Writer (Word), Calc (Excel), Impress (PowerPoint), Base (Access)
Compatibility: Opens and saves Office 2013 files (.docx, .xlsx, .pptx)
Available for: Windows, Mac, Linux – no license needed