From beginning to end, what are you trying to do?
Does that Dropbox folder belong to you?
Are you trying to download the FBX from the Editor?
If not, how are you intending to download the FBX?
Are you trying to load the model at runtime in PlayCanvas and the model is hosted elsewhere?
Is this what you are trying to do? Load 3D models at run time from a PlayCanvas application
I suspect there is a language/terminalogy problem here.