After you export files from Lightroom to a folder, some of the files have a file size of 0 (zero) KB. You cannot open them.
When exporting, Lightroom's first step is to create a file place holder. If you export to a Dropbox synced folder, this place holder is immediately synced to your cloud storage. When this other process takes over the file, it becomes locked. Lightroom can't add new data to it, so it stays a zero-byte document.