Explorer
In File Explorer, right-click the file and select Send to > Kindle. The 1.1.1.260 package description also says you can select more than one document and choose Send to Kindle.
Send to Kindle for PC is Amazon’s free Windows application for personal documents. Version 1.1.1.260 runs on Windows 10 or later. After you install it and sign in, you can right-click a file in Explorer, drop it on the app, or choose Send to Kindle in a print dialog. Amazon’s help pages state there is no additional cost (0 USD).
Hover the panel, or turn on Show the send. A PDF and an EPUB move onto the paper display. On Windows, the real paths are Explorer, Drop files here, and the print dialog.
Personal documents and other files can be added to your Kindle library and read on a Kindle or in the Kindle app, with reading progress, notes, highlights, and bookmarks.
Download and install the Send to Kindle app for Windows.
By using Send to Kindle, you agree to the terms.
Supported file types: PDF, DOC, DOCX, TXT, RTF, HTM, HTML, PNG, GIF, JPG, JPEG, BMP, EPUB.
In Explorer, right-click the file you want to send and select Send to > Kindle.
or
Open the Send to Kindle app to upload by drag-and-drop.
The Windows page’s line for the result is “Send once, read anywhere.” Files in the library can be read on a Kindle or in the Kindle app with reading progress, notes, highlights, and bookmarks.
In File Explorer, right-click the file and select Send to > Kindle. The 1.1.1.260 package description also says you can select more than one document and choose Send to Kindle.
Open the Send to Kindle app and drop files on the area labeled Drop files here. The Windows page gives this as the alternative to the Explorer menu.
The package description says Send to Kindle appears in the print dialog of a Windows application. An earlier Amazon page for the PC app said those documents are delivered as PDF.
Figures below are from the winget manifest for Amazon.SendToKindle and from the Send to Kindle hub. Windows 10 requirements and version 1.1.1.260.
Short answers sit here. The full wording, including sync and sign-in, is on the Send to Kindle FAQ.
Install the app, sign in, then use Explorer, Drop files here, or the print dialog.
MOBI is not on the current Windows format list. The installer still registers that extension.
Notes, highlights, bookmarks, and last page read sync on archived documents converted to Kindle format.