I need to open this pptx-file. Can anyone help me with that? http://www.filedropper.com/testfileq9
Made in Microsoft PowerPoint 2010 and cannot been seen or open with any applications...
If it can't be open, how can I fix the file.

pptx file is power point presentation file I can't download the file for security reason but what operating system are you using? do you have microsoft powerpoint on your device/computer
I would suggest you try and convert pptx file to pdf or any other file format you want

this website will help you do that

I downloaded the file but could not open it with PowerPoint 2010.
A healthy Office 2010 file, be it PowerPoint, Word or Excel can be renamed with a .zip extension. The elements of the file can then be inspected by opening the zip file.
This does not work with the file you attached.
It's either not a PowerPoint 2010 file, or it has become corrupt in some way.
Sorry, but that's the best I can do.
You can still try to recover the file using PPTX Repair Kit. I very much hope that this will help you. http://www.pptx.repair/
Or contact the party that created the file and ask for another, clean and verified, copy.