Here is a list compiled according to the product feature, response time, cost-effectiveness, and user interface.

1. SwifDoo PDF (Windows, Mac, iOS, Android)
2. Wondershare PDFelement 
3. Soda PDF 
4. PDF-XChange Editor 
5. Adobe Acrobat Reader 
6. Adobe Acrobat Pro 
7. PDF Expert
8. PDFescape
9. Drawboard PDF
10. Microsoft Edge
11. Kami

1. SwifDoo PDF

SwifDoo PDF is a light and multi-purpose best PDF annotation app for Windows, Mac, iPad, iOS, and Android. With a set of annotation tools, it enables you to freely draw on a PDF, highlight, underline, strikethrough text, insert caret, add sticky notes and stamps, and more. When performing a task, you’ll find the application always starts and runs smoothly without any log.

Even for a heavy file with 600 pages, the software allows you to view and annotate it quickly. All in all, it is recommended as the best PDF annotator.

Here is an instruction on how to use the app to annotate a PDF file.

Step 1: After free downloading the installer, directly launch the program and open your PDF file with it. You can click Open in the home tab to import your file or drag and drop it into the window;

Step 2: Go to the Annotate menu on the top and choose any of the provided features to start annotating the PDF;

Step 3: When you're done, click SwifDoo in the top left corner and choose Save as to save the changes.

If necessary, the PDF annotator enables you to insert, replace images, and add headers and footers in a PDF. You can use its PDF encryption feature to encrypt your PDF with a password. The handy software also lets you convert PDFs to images or other formats. In a word, SwifDoo PDF is a one-stop shop for a variety of PDF-related needs. With respect to the user interface, it resembles Microsoft Word, which is designed for creating a great user experience.    

Pros:

• Rich annotation features
• Cross-platform availability (Windows, Mac, Android, iOS)
• Lightweight, and fast
• Clean and simple user interface
• Cost-effective ultimate solution to almost all PDF tasks (PDF editing, PDF conversion, PDF compression, etc.)

Cons:

• No online version.

2. Wondershare PDFelement

PDFelement is an all-inclusive solution to PDF issues. As the best PDF annotator, it has an array of free annotation features to highlight text, add text boxes, insert shapes in PDFs, and more. When the need arises, it enables you to use the callout tool to markup an image, or add an attachment to a PDF document. This software also has good PDF editing and OCR features. You can download the best PDF annotation app for Windows, Mac, iPad, iPhone, and Android.

Pros:

• All necessary annotation features, and other advanced features such as extracting data from form fields in a PDF
• Batch files processing ability
• Smooth program execution

Cons:

• User-unfriendly interface for annotation and some other features
• Watermarks added on a PDF document when saving changes during a free trial. Only half-year, one-year and perpetual subscription plans without short-term plans

3. Soda PDF

Soda PDF is a desktop best PDF reader with annotation features for Windows and Android. To be specific, this PDF software has both web and desktop versions. It is on the list of the best PDF annotators, enabling users to markup, whiteout text, add attachments in a PDF, and much more. Besides, the app has a handy measurement tool that allows you to measure the length, perimeter, and area of elements in a PDF file.

In addition to PDF annotation, Soda PDF offers a lot of other features that enable users to edit, merge, split, and rotate PDF documents. However, concerning response time, the program runs a little slowly. Despite this, it is still a top pick for handling PDFs overall.

Pros:

• Powerful annotation features including measuring objects in a PDF file
• Intuitive navigation bar
• Offer a web version

Cons:

• Relatively slow program response

4. PDF-XChange Editor

PDF-XChange Editor is another software that deserves to be named the best free PDF annotator with a free version. Its free version is equipped with a range of tools for users to add text boxes, sticky notes, highlights, strikethroughs, or other comments to PDF files. If necessary, you can import or export annotations from or to a file. The software’s pro version can convert, edit, and organize PDFs. It’s undoubtedly qualified. 

Pros:

• A comprehensive PDF annotation tool for various annotation tasks. 
• Free version provided
• Fast to use

Cons:

• User interface to be improved
• Output documents will be watermarked after some tools are used in free mode

5. Adobe Acrobat Reader DC

When it comes to the best free PDF annotation app for Windows, Mac, and mobile phones, Adobe Acrobat Reader DC is always worth mentioning. Included with the family of Acrobat software, it is well-known for its free service. You can utilize the program to add nearly every type of comment to your PDF documents, from text and pop-up notes to shapes and attachments. Signing and printing PDFs for free are also its capabilities.

Adobe Reader is sure to be the best free PDF annotator that both Windows and Mac can run.

Pros:

• View, comment, sign, and fill out PDFs
• Compatibility with both Windows and Mac
• Allow toolbar customization

Cons:

• Heavy program compared with many counterparts

6. Adobe Acrobat Pro DC

Adobe Acrobat Pro DC comes from Adobe. Inc, the inventor of PDF, which you must have heard of before. With a suite of great tools and outstanding performance, Acrobat Pro is sure to meet your needs for PDF annotation. You can employ this PDF annotation software app to complete almost all annotation tasks, including adding text boxes and shapes to a PDF.

As a full-fledged PDF management application, it also lets you edit, convert, compress, and combine PDF documents. Even so, many people feel Acrobat Pro is not pleasant to use because of the costly subscription and sophisticated interface design.

Pros:

• Excellent PDF annotation and other features
• Smooth and rapid program response

Cons:

• Cluttered interface
• Only yearly plan available, at about $180 (The price is higher than the perpetual plan of SwifDoo PDF)

7. PDF Expert

PDF Expert is a popular PDF management tool compatible with macOS. With its annotation feature, you can draw on a PDF, highlight text, and add notes and stamps to a PDF document. If needed, the desktop best PDF annotator allows you to export all annotations as a single file. On top of PDF annotation, the software can also edit, combine and sign PDFs. Like every other PDF application, PDF Expert has its downsides. For example, it has no advanced features such as OCR.

Pros:

• Common annotation features
• Easy-to-use UI
• Available on iPad and iPhone

Cons:

• Not work well with large PDF files

8. PDFescape

Without a PDF application installed on your device, annotating PDFs can be tricky. Thanks to the online free PDF annotator app - PDFescape, you can add annotations to PDF files through a web browser. The feature-rich PDF software, PDFescape, provides a quick and free way to deal with PDFs. It not only allows you to add comments but also enables you to insert form fields into a PDF file.

You can organize pages and add additonal elements. From the best free PDF annotation software, you can download an annotated PDF for free without a watermark.  

Pros:

• You can annotate a PDF for free with the online tool. 
• Easy to access through Chrome, Firefox, Edge, etc

Cons:

• Only 5 fonts for added text
• Old-fashioned UI design

9. Drawboard PDF

Drawboard PDF is reviewed as one of the best free iPad/iOS/Android PDF annotation apps for students or research works. It's praised for the lightweigt program, modern and eye-caring interface, and amazing annotation options. It is also available on Mac, and the web. It allows teams to do real-time markups and collaborate on design reviews. It's designed to simplify students' study and save their time with powerful performance and productivity tools. 

Pros:

• Pressure-free annotating that supports shape recognition.
• The free PDF annotator software is available on iPad, iPhone, iPhone, Mac, and online. 
• Sync documents across devices using your account.
• Work with stylus if needed.

Cons:

• Sometimes lag.

10. Microsoft Edge

If you look for a free app to annotate on PDFs, Microsoft Edge is the basic solution. For those who don't need complicated and advanced annotation or editing functions, the web browser is enough. Edge equips itself with a PDF viewer, highlighter, text adder, line drawing tool, and note taker. If you run a Windows, you will save effort downloading third-party apps.

Additionally, Edge also offers a read aloud feature for you to listen to PDF text. The best lightweight PDF reader with annotation tools is good for daily document feedback and reviews. 

11. Kami

Kami is one of the best free PDF annotation tools that can work with Google Drive and Chrome browsers. To use the app, you need to connect it to your Drive from Google Workspace Marketplace or add it to your Chrome from Chrome Web Store.

Kami has the ability to markup and erases content, and draw and leave comments on a PDF. You can add audio or video comments to PDF files. Furthermore, this app enables you to easily split, merge, and sign documents. If you need more complicated features such as PDF conversion, it's better to turn to a more comprehensive PDF application.

To Sum Up

A great app for annotating PDFs bring an excellent experience. The 8 top PDF annotation apps and the Kami Chrome extension help you be more productive with your work. Choose one according to your needs and budget to effortlessly add and delete notes, stamps, highlights, and a variety of comments on your Windows, Mac, or online.