This, in my view is a moral issue. It depends on where you are dowloading the book. If the book is being given free by the author or by someone who has the author’s authority to download it, then it is ethical. Also there are other books whose copy rights may have expired and you may access the books without worrying about the ethics. However, there are many books that are illegally uploaded on the internet without the authors knowledge and consent. In that case, I would say it is unethical to download the book and use it without the consent and permission of the author. However it also happens that you may want a certain book and be prepared to buy it, but it may not be readily available and you might be forced to download the copy. Quite frankly it might be argued that downloading a pirated book might actually be stealing. My suggestion is if possible, trying to contact the author or publisher of the book that you download if it is on a site that is not associated with the author or publisher and ask for a permission to use it. I have done that before and I have actually got the permission. This is not an easy subject because ethics are subjective but talking about copy right issues is more appropriate.
So, don't worry, I did not come up with this theory. Well, there is no problem with PDF books. >>>They are societies expensive. The price would be more comfortable if it was less than 100. >>For example, this paper is 10.00 so 200 is the minimum price. Of course, we would have to take into consideration the availability of such an alternative at the time, but still. >Also, the PDF file format is not as intuitive as the PC printer. >>So, for example, in Microsoft Word, a document can be saved as a PDF file and then imported into Word with a single click. However, in Microsoft PDF-to-A converter, such a thing is not possible. And you cannot even open and edit any of the file type, which is probably the main goal of such converter.