Hi Pineau,
Thank you for contacting Syncfusion support.
Documents cannot be opened directly from Asset folder in UWP platform. Please follow the below steps to open documents in UWP platform.
1. Add the document (Excel document) into Embedded Resource.
2. Create stream for the document.
3. Open the stream using Workbook.OpenAsync(stream) method.
We have also prepared a sample for this which can be downloaded from the following location
Regards,
Sridhar S.