If you see a note that the book is archived (vs. the Download or Send to Kindle buttons), the publisher has archived the book and it is no longer accessible via NetGalley. After the book is archived you will not be able to download the file via NetGalley at all.
However, you can still provide Feedback or review the book, even after it has been archived.
Many publishers provide archive dates in advance. If you plan to read or provide feedback on a book, we highly recommend that you download or send the book to your Kindle soon after you are approved.
Any copy downloaded to your computer, iPad, or Adobe-compatible reader will be available until you reach the Adobe Digital Editions expiration date. If you used the Kindle option to send a copy of the file to your Kindle (app or device), that file will not expire, and will continue to be available to you.
For more information on expiration and archive dates click here.