Portable Document Format (PDF) is a popular file format for office documents which was developed by Adobe, representing a document in PDF format will make it looks more professional and official. Converting documents to PDF format is very easy, you can turn your Word, Excel, PowerPoint, OneNote, Visio, HTML, TXT or any other documents into PDF format with freeware like doPDF, Bullzip PDF Printer, CutePDF Writer, etc. However, to view, edit, convert or manage PDF files, you'll have to install special software designed for these purposes, many of them are not free, for example, Adobe Acrobat Pro DC will cost you more than 400 dollars. In addition, viewing pdf files will occupy much memory, rendering the text with different fonts with anti-aliasing enabled and displaying the pdf file on your device may even cause your system frozen, or make your device( such as iPhone, iPad, Samsung Galaxy S6 ) too hot. Some pdf viewing software and app is not secure, for instance, people have found more than 50 security holes in Adobe reader. Some malicious PDF files even contain malware which may infect your system. When you receive a pdf file with Email, WhatsApp, Facebook Messenger, Snapchat, Line, Telegram, Skype, ICQ, or other software, download a pdf file from the Internet( including torrent sites, online chat rooms ), you should always scan it with an antivirus software before opening it to keep your system secure.