> Let Office do the heavy work for you. Word and Excel have extremely complete object models, available via COM Automation ... You have a web-based application that’s needs to output existing Word files in PDF format. Here’s how I would implement that: a few lines of Word VBA code loads a file and saves it as a PDF using the built in PDF exporter in Word 2007. You can call this code directly, even from ASP or ASP.NET code running under IIS. It’ll work.

It'll work until it doesn't [1]. Like if you want to do two things at the same time.

> Considerations for server-side Automation of Office

> ...

> Reentrancy and scalability: Server-side components need to be highly reentrant, multi-threaded COM components that have minimum overhead and high throughput for multiple clients. Office applications are in almost all respects the exact opposite. Office applications are non-reentrant, STA-based Automation servers that are designed to provide diverse but resource-intensive functionality for a single client. The applications offer little scalability as a server-side solution ... Developers who plan to run more than one instance of any Office application at the same time need to consider "pooling" or serializing access to the Office application to avoid potential deadlocks or data corruption.

And if you want to use this functionality to service the requests of anonymous users, make sure to read up until "[u]sing server-side Automation to provide Office functionality to unlicensed workstations is not covered by the End User License Agreement (EULA)."

[1] https://support.microsoft.com/en-au/kb/257757