Write an article based on this "Open the folder that contains the .OBJ file. Right-click the file. Click Open with. Click Paint 3D."
An easy way to do this is to press ⊞ Win+E to open File Explorer, then browse to the folder where it’s saved. A context menu will expand. It’s at the top of the menu. Another menu will expand. This opens the file on your PC. .OBJ files are also supported by Adobe Photoshop and Mixed Reality Viewer. If you have and prefer one of those apps, select it instead.