Do you use PDF files or read documents in PDF format? Even if you don’t use them regularly, I’m sure that you’ve come across a PDF file or two in your lifetime. They’ve been around for over 20 years and can contain links, buttons, form fields, videos, audio and more.
Reading PDF files can be done in many applications, but a lot of those apps don’t include annotation tools. In this post we’re going to focus on apps that do allow you to annotate PDF files. Whether you want to add text notes, bookmark a section, highlight or underline text, these free tools will allow you to do just that.
A.Nnotate
A.nnotate is great for those wanting to annotate images, PDF files and Microsoft Office documents online. With it, you can add text notes, highlights and strikethrough. The collaboration feature with threaded comments and the ability to add tags to notes, makes A.nnotate great forfreelancers, small teams and even businesses.
PDFescape
PDFescape is not only great for editing and annotating PDF documents, but you can also create new PDF forms, fill out PDF forms, password protect your files and share documents. With PDFescape, you can add text comments, shapes, images, sticky notes, and highlights to documents. Check out many other advanced features PDFescape has.
GroupDocs 
GroupDocs is an advanced document collaboration tool that includes annotation features for PDF and Microsoft Office documents. You can add comments and mark up your text in real-time, much like documents in Google Drive. GroupDocs has 3 paid plans, and a free 14-day trial plan that lets you try before you buy.
PDFOnlineReader 
PDFOnlineReader is not just for reading PDF files online. You can also add annotations, add hyperlinks and bookmarks, and highlight text. It’s a lot simpler to use and it’s great for those who want to annotate quickly, save and download their files. It also offers PDF desktop products for batch PDF creation and converting PDF files to other formats (and vice versa).
Crocodoc 
This online document viewer lets you upload and annotate PDF files, Microsoft Office documents and images. With Crocodoc you can add text comments and drawings, highlight,and strikeout text on your documents. You can also use it to fill out PDF forms, collaborate in real-time and much more.