As of the 22nd of July 2011, it's taken me about 8 hours to finally do it, and it was an extremely painful process that involved a lot of research and trial/error. I hope that this post can spare you some of that pain.
Why Your eBook is Stuck in Adobe Editions
So, a little bit of back story to help you understand why this is so convoluted: eBooks which are sold nowadays often come encrypted with Digital Rights Management (DRM). This is a way for publishers to restrict the way in which you use your downloaded content - primarily with the goal of preventing piracy.
The downside, however, is that it makes it incredibly hard to do what you want with your files - for example - putting them on your iPad or Kindle.
When you buy PDF eBooks from certain online retailers (in my case, whsmith.co.uk) they are advertised as "PDF" - but they aren't really. You get a download of an ACSM file, which you have to open through Adobe Digital Editions to download your eBook. Once it's downloaded, it's locked into Digital Editions, and there is no compatibility with iPad or Kindle.
The only way to get the file onto your iPad or Kindle is to get it out of Adobe Digital Editions, so it functions as a normal PDF or EPUB file.
Digital Editions saves a PDF file to your hard drive (/Documents/Digital Editions/Your_eBook.pdf) but if you try to open this file with any application other than Digital Editions, it will throw up errors or say that the file is corrupted. Usually something like:
"The file [xxx] could not be opened. It may be damaged or use a file format that Preview doesn’t recognize."
This is because the PDF file is encrypted with Adobe Adept DRM, which is surprisingly difficult to get around. We need to remove the DRM from the file in order to transfer it to a Kindle, or import it into iTunes. Unfortunately, most DRM removal software will not be able to handle this type of file. Trust me, I tried a lot of them.
It's worth noting, at this point, that it is completely legal to remove DRM from a file which you have purchased. You aren't doing anything wrong unless you're removing DRM from a file which you don't own. So let's get started.
How To Remove Adobe DRM from Your Ebook
First: You're going to need a PC. Mac users (I'm one of you), there is no way to do this that I've found on a Mac, so you're going to need to find/borrow/steal a PC, or set up a virtual machine with something like VMware Fusion. Here's the step-by-step:
Step 1: Install Adobe Digital Editions on Your PC
If you're already on a PC, you've probably already done this. Mac users, get set up in a PC environment. You need to download Adobe Editions, activate your computer with an Adobe ID, then download your ACSM file, and get the eBook into Digital Editions.
You MUST have Adobe Editions installed and activated on a PC before proceeding to the next step. The DRM removal app used later will not work if you copy your PDF file from your Mac to PC and try to decrypt it.
Step 2: Install ePubee
The little application that does all the magic is called ePubee. Get it here. The trial version of ePubee is free, and will let you remove DRM for 3 eBooks before you need to upgrade to the paid version. No watermarks or limited features, which is nice.
Step 3: Remove DRM
Click on the "input" button and navigate to where Adobe Editions has stored your encrypted PDF file, and select it. On Windows this is in My Documents > My Digital Editions > Your_eBook.pdf
Click on the "output" button and select where you want to save the new (DRM free) PDF file which you can put onto an iPad/Kindle. I just select My Documents for this.
Click on "Remove DRM"
You should get a success message, and a shiny new PDF file titled Decrypted_Your_eBook.pdf located in My Documents.
Step 4: Back to The Mac
If you're a Mac user, that's all you needed the PC for. You can now upload the file to Dropbox or email it to yourself, and go back to working on your Mac. PC users, skip this.
Step 5: Optional File Conversion
Both iPad and Kindle will read your new PDF file, now that it's DRM free, however both will sometimes render the file better if you convert it to their preferred format. For iPad this is .epub - for Kindle this is .mobi.
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This, thankfully, is rather easy. Head over to Zamzar, upload your file, select the format you'd like, and put in your email address. In a few moments (usually under 3 minutes) Zamzar will email you your converted file. There are other tools, such as Calibre (for Mac) which will do this straight on your computer rather than through the web.
I have found that conversions (to .mobi in particular) don't work well for books which are graphics heavy. It does the text no problem, but any special coloured info-boxes (typically found in non-fiction books) or images just come out as a blur.
I really hope this worked for you, and more than anything I hope I saved you the hours of screwing around with Python scripts through command line interfaces that I went through. DRM is no fun for people who just want to use the files they've paid good money for. Adobe, unfortunately, always love to make everyone's lives more difficult at every possible opportunity.
Here's to their eventual demise.