I recommend PDF-XChange Viewer for this task.
It is a gratis (no cost) pdf-viewer for windows that also happens to include extensive metadata-editor functionality.
It has no ads for third-party programs. It does have a tiny static promo for their PDF Creation Tools program, but it is hardly noticeable. There is nothing deceptive in the program at all, and my experience is that the company that develops this software is very reputable.
The downloads page includes a link to download a portable version that does not require any installer.
To edit metadata, simply go the File menu and select Document Properties (or press Ctrl+D). A dialog box will pop up in which you can edit the metadata. There is even an Additional Metadata button in that dialog box that will allow you to quickly edit the Document Title, Author, Author Title, Description, Description Writer, Keywords, Copyright Status, Copyright Notice, and the Copyright Info URL tags. It's quite impressive.
I have successfully used this application many times, and I can recommend it as a high quality piece of software.