Portable Document Format (PDF) is one of the most widely used open standard for documents. It is a complex format that is designed for publishing output but not a whole lot of further modifications. It became popular for a whole lot of reasons, but editing was probably not one of them.

It is possible to convert almost any document format into a PDF and with a much smaller file size. Due to its smaller file size, it makes it easier to share large documents and be sent over emails. Another advantage that PDF files have is that it renders pretty much the same on all machines and platforms.

Rendering the PDF is probably one of the easiest and for that reason there are a whole lot of free PDF readers that you can use.

Adobe Reader (formerly Adobe Acrobat Reader)

Adobe Reader is a free product from Adobe which allows you to view and print PDF documents. It provides very minimal file modification capabilities. You can get the reader for almost all platforms, including MS Windows, Mac OSX and various Linux distros.

Google Docs

Google docs can be used to view and print PDF documents as with the Adobe Reader but without having to install the reader locally. It does not provide any capabilities to modify the document. Being an online reader has it advantages, as it can be used with some browser extensions to automatically open, view and save the PDF files as you browse and it eliminates the need to install an reader locally on the machine.

But as with any cloud computing services, you need to be online to use the reader. So, if you need offline viewing capabilities, the Adobe Reader is probably the better option.

If you use Mozilla Firefox for browsing, there are many addons that you can use to open documents, including PDF automatically in Google Docs. Do a search for Google Docs in Mozilla addons. If you use Google Chrome or Chromium, you can try this extension (Docs PDF/PowerPoint Viewer) which works well for me.

Okular (For KDE)

If you are on KDE desktop, It comes with Okular which is an universal viewer for many different types of documents, including PDF.

Evince (For Gnome)

On Gnome, you should be able to use Evince which is the universal document viewer.

Searching inside a PDF

All the readers mentioned above support searching for text within a PDF document which is open in the reader. If you do have all your documents uploaded to Google Docs, you can use the google search to perform searches inside your PDFs (or converted documents). But what if you want to perform a search over several PDF documents that are on your local hard disk?

If you are on Linux, you can try pdfgrep, which as it says works very similar to grep. You can search multiple PDF documents with a similar syntax to grep.

pdfgrep -in <searchterm> <file pattern>

Another option will be to use the pdftotext command which converts the PDF to text and then do a grep over it.
find /<path-to-the-folder> -name '*.pdf' -exec pdftotext {} \; | grep "<your query here>"

Secure PDF file

I cannot imagine why you would ever want to open or read a file for which you don’t have the password for, but it could sometimes happen. Give pdfcrack a go. I have had some success with it in the past.

But as i mentioned earlier editing a PDF requires some different approach, read the post How to edit a PDF document if you are looking to edit some PDFs…..