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Choose the file and your device. We'll show the right route, with no upload and no DRM removal.
The answer
Don't copy it over the cable: use Send to Kindle, which converts it and adds it to your library.
See how to send it to Kindle →DRM is not a file error. We do not remove it or recommend tools that do: your store, publisher and licence determine where an ebook can be read.
If you have a text PDF, convert it here. If the problem is an EPUB that will not open, validate it with EPUBCheck in your browser. No file leaves your device.