By trying to open a password protected PDF file on the Amazon Kindle Fire, you will probably see a warning box message that says "Invalid Item - This item is protected with DRM and cannot be read on your Kindle. Please remove the item from your device and download it again or purchase a copy from the Kindle Store." And many people have encountered the same question. To read the password protected file, you’ll need a third party application that opens password protected PDF files.
Unlike these Kindle incompatible formats, protected PDF files could be transferred to Kindle successfully. The only issue is you cannot open it on Kindle. The one stop solution of removing PDF protection for Kindle is using PDF Password Remover for Mac. It can remove the restrictions on printing, editing, copying in seconds and provides 2 decryption modes: Batch decryption mode and Right-click decryption mode.
Step 1: Download and install this PDF Password Cracker for Mac. Launch it and you will see its main interface.
Step 2: There exist two ways to add the protected PDF files. One is go to "File -> Add File(s)" to locate your files; another is directly drag-ang-drop the PDF document to its main window. After adding file(s), you will see its basic information as follow:
Step 3: If you have the correct Open Password of the file, you can click the lock icon next to the PDF file and type the Open Password to access the file for reading.
To customize the output destination from "Browse" button on the screen. Directly click "Select..." button to specify the folder.
Step 4: Unlock and Edit Secured PDF on Mac
On right bottom of the program, hit "Start" button to remove protected PDF files for Kindle. After doen, you can hit the "Open" button to view the output file with any PDF viewer.
Now attach your Kindle to Mac computer and transfer the unprotected PDF files to it using the USB cable. You can refer to How to Transfer PDF eBooks to Kindle.