Windows Bulk Letter Generator -- 2

I need a small, stand-alone Windows application that lets me create placement or appointment letters one at a time or hundreds at once, entirely offline. The workflow I have in mind is straightforward: I select or load a template, point the program to a list of recipient details, and the software outputs perfectly formatted letters in both PDF and DOCX.

Key expectations
• Runs natively on Windows 10/11 with a simple installer or single executable—no internet connection, log-in, or cloud licensing involved.
• Supports individual generation for quick one-offs as well as true bulk mode for large batches.
• Exports two files per letter—PDF and DOCX—while preserving fonts, margins, headers, and any embedded logos.
• Lets me choose where the finished files are saved and automatically names them from a field I specify (for example, employee ID or full name).

Deliverables
1. Final compiled Windows program.
2. Source code with brief build instructions.
3. Short user guide that shows how to load a template, feed data (CSV, Excel, or a format you recommend), and run both modes.

Acceptance criteria
• Application opens and runs offline on a clean Windows machine without extra installs.
• A single test letter and a batch of 500 letters render identically in PDF and DOCX.
• No data is transmitted outside the local system.
• Bulk generation completes in a reasonable time frame (e.g., 500 letters under two minutes on an average laptop).

I’m flexible on the tech stack—.NET, Python packaged with PyInstaller, or any Windows-friendly toolkit is fine—as long as the final result meets the above points. Let me know how you’d approach this and any questions you have about templates or sample data.