Thanks for providing us some details.
Well, it is strange that you are finding this issue. Generally, if your solution (with similar code snippet and Aspose.Cells for .NET v8.7.2 assembly) works fine on your local machine, it should equally work fine on your server. Please double-check the code (line by line) snippet used on the server project, it might be a spam in your code which are added accidently to your output Excel and PDF file (so you are getting “ff” in the first row in the output file formats) or you might be using different version of Aspose.Cells component. I am afraid, we have no clue to your mentioned issue unless we could have your project (runnable) to evaluate and to reproduce the issue on our end. If in any case, you could not evaluate your issue and the issue is still reproducible, kindly do provide us a simple sample project (runnable) to show the issue, we will check it soon.
PS. Also, using the component in evaluation mode (without using/setting a valid license) does not embed or input “ff” to your output files, it would add Evaluation watermark to the output Excel/PDF file format.