PDF (Portable Document Format)   files are everywhere. It is used as an excellent way to share information across   platforms and has been around for a long time (17 years is an eternity in   computing terms). PDF is also an open standard (ISO 32000), which means that   there are no barriers to entry.
The beauty of PDF files is that   they look like original documents but are also searchable. This means that you   can find text within a file, without needing to read the whole   document.
One of the myriad of used is for   manuals; imagine having all your manuals, not just for electronic items but,   household appliances, garden equipment, whatever, all in one place. Not only can   you find them but, you can find the information you are looking for within   them.
I'm not attempting to sell you on   the idea of PDF (with over 150 million PDF files publicly available, the jobs   already been done).
There are loads of PDF readers for   iOS; some better than others. The difference with PDF Expert is that it allows   you to read (obviously) and annotate PDF documents, including highlighting,   making notes, and drawing. You can even add a page but not create a new blank   document. I am so impressed by this App that there is a workaround to this; here   is a link to a blank page for you to download (with an   option to also have a confidential stamp). 
Have you ever been irritated by   someone (invariably a financial institution) sending you a PDF form to complete   and then telling you that you cannot save the completed form; you need to print   it out and then keep a copy for your records. PDF Expert allows you to complete   the form and save it, and even sign it.
A word of caution though; the   versions for the iPad and iPhone are not the same. The iPad version is the one   with bells and whistles.
Now for the ACIDS   test:
Aesthetics
The   aesthetics of individual documents are, obviously, nothing to do with PDF   Expert. As far as the App is concerned, it is as one would expect a listing of   folders/files, with the option of picking any one (as well as moving) and then   opening in a whole window.
One small word of caution though   when using Dropbox (and that is very much a cornerstone of making this a killer   App); remember to make sure that (if you modify a document), you resync to   Dropbox.
Cost
It's not   cheap but, in my opinion, is well worth the cost.
Indispensability
If   you want to have all your PDF documents in one place and be able to annotate   them then, this is the only choice. You should use this in conjunction with   Dropbox which will ensure that you have the current version both on your iPad   and your PC.
Dependability
Works   100% of the time (so far).
Security
As far as   the PDF Expert App is concerned, the potential issue is that you store your   signature and then leave your iPad lying around. As far as synchronising with   Dropbox is concerned, this is covered here.
ACIDS Test Rating: 
 You've just got to have   it!
To find out how the   ACIDS   Test works, click here.