This article was updated on 16 Dec 2025
If you’ve ever wondered how to annotate a PDF, the process is simpler than you think — and it can completely change the way you read and edit documents. Whether you’re editing for work or study, annotation tools help you highlight, explain, and share ideas right in your PDFs.
What is PDF annotation software?
PDF annotation software lets you add notes, comments, highlights, and other visual elements directly to a PDF file. It gives you a way to make your documents more descriptive and actionable without changing the original content.
With MobiPDF, you can annotate your documents quickly and efficiently. You can add comments, highlight text, insert shapes, or even attach more files to specific pages. This means you can explain your ideas clearly, mark key details, and make sure nothing important is missed.
Typical annotation options include:
Adding comments or sticky notes to existing content
Highlighting, underlining, or striking through text
Inserting stamps and signatures
Drawing shapes such as arrows, rectangles, or circles
Attaching images or supplementary files
Adding polygons and connected lines for more advanced markups
A good PDF annotation tool also lets you make immediate PDF text changes. This includes the ability to highlight, strikethrough, or underline the content you need. This is very useful when you need to emphasize parts of the content that are important for context, and need to draw attention to them. They can also simply serve as a reminder for what you need to focus on in later edits.
How do I annotate a PDF?
Many users also ask, “how can I annotate a PDF document effectively?”— and the answer is by using the right annotation tools built into MobiPDF.
Here’s how:
Open your file in MobiPDF.
Go to the Comment tab — this is where you’ll find all the annotation tools.
Choose the annotation type you need (Notes, Highlight, Shapes, Polygons, Connected Lines, etc.).
Click anywhere on the document where you want to add the annotation.
Type your note, draw your shape, or pick the highlight colour.
This simple process works on both desktop and mobile, and all annotations are automatically saved and synced with MobiDrive, making sure your edits are consistent across all platforms.
Adding comments, so everything is crystal clear
If you’re wondering how to annotate a PDF, the answer is to start with the basics, including the option to add comments to select parts of the PDF content. This also covers how to add notes to PDF files – a great way to explain ideas or clarify sections for your collaborators.
Comments also reduce the risk of misunderstandings and lead to a more productive workflow with collaborators.
What are the advantages of annotating a PDF?
Annotating a PDF document lets you provide added context and clarity for your readers or collaborators. You can annotate PDFs by adding text boxes, freehand drawings, and file attachments.
Examples of annotation features you can use:
Highlight key data in a report.
Add sticky notes to explain a chart or figure.
Insert comments next to contract clauses for review.
Attach related documents or images for easy reference.
One of the key things when annotating PDFs is improving your communication with others who might use the same document.
How Can I Add a Signature to a PDF?
You can sign with a custom signature or you can also add a digital signature, available under the Protect tab in MobiPDF.
So, you can add two types of signatures:
Handwritten or Predefined Signature:
Go to Fill & Sign → Quick Sign.
Create or upload your signature.
Save it for future use.
Digital Signature (with Certificate):
Open Protect → Digitally Sign.
Use your unique digital certificate.
Save the document to secure it with encryption.
Digital signatures make sure your document is authentic and prevent unauthorized changes — which is essential for contracts and official forms.
How to annotate PDF files – a step-by-step guide
Annotating a PDF helps you add context, comments, and explanations directly to your document. With MobiPDF, you can easily insert notes, highlight text, or add visual markers to make your content clearer.
Here’s how to add a comment using MobiPDF for Windows:
Open your file with MobiPDF.
Click on the Comment tab.
From the presented options, select Notes.
Click anywhere in the document where you need to insert a comment.
Type the content in the pop-up note and click anywhere outside the note to save it.
For your convenience, Text Annotation can also be accessed from the Home tab.
How to work with texts when annotating a PDF
Sometimes, plain text alone might not be enough when annotating a PDF and you may have to use additional techniques like highlighting, underlining, and more.
Here is how to annotate a PDF by editing text in MobiPDF:
Click on Select within the Home tab.
Select the text you want to change and a floating annotation menu should appear automatically.
From the dropdown menu, choose the annotation option you need.
You can cut, copy, or edit the selected text, then adjust its style using Edit Text. You can also hold SHIFT to draw perfectly straight lines when using drawing tools. Change fonts, colours, size, or even rotate the content from the Editing Tools tab to match your layout needs.
How to add stamps to a PDF
Stamps are a quick way to mark a PDF as Approved, Reviewed, or Draft and guide collaborators at a glance. In MobiPDF, you can insert pre-made stamps in seconds and adjust their placement.
To add a stamp:
Open your file with MobiPDF.
Choose Comment, then Stamp.
Click on the spot where you want to position the stamp annotation.
You can use the dragging points to resize the stamp, move the stamp around within the document, and left-click and drag it to the desired position.
How to add attachments to a PDF
You can easily attach additional files to your PDF in MobiPDF – perfect for adding references, images, or supporting documents.
To attach a file:
Open your document in MobiPDF.
Go to Comment → Add Attachment.
When the directory window opens, select the file you want to attach.
Click Open to confirm your choice.
To view all attachments:
Go to the side panel.
Choose Attachments.
FAQs
How to annotate a PDF file?
To annotate a PDF file in MobiPDF, open your document and use the Comment tools. You can add notes, highlights, underlines, or shapes directly on the page to make your PDF more interactive and easier to review.
How to annotate a PDF on a computer?
You can annotate a PDF on a computer using MobiPDF by opening your document, selecting the Comment or Highlight tool, and clicking anywhere on the page to add your note.
How to annotate a PDF on a laptop or tablet?
If you're using a tablet (Android/iOS), open the file in the MobiPDF mobile app and use tools like Highlight, Underline, Notes, or Shapes. Note that the mobile interface of MobiPDF is optimized for touch input so the workflow is a little different than on a PC.
How to add notes or highlights to a PDF document?
To add notes or highlights to a PDF document, select the Comment or Fill & Sign tool from the menu in MobiPDF. Click on the area you want to mark, type your note, or choose your preferred highlight colour to emphasize important details.
How to annotate a PDF on Mac?
MobiPDF does not currently have a Mac version, but you can annotate your PDFs on Windows or Android/iOS using the same tools and synced files via MobiDrive.
How to annotate a PDF on Windows?
To annotate a PDF on Windows, open your file with MobiPDF and go to the Annotate tab. Choose your desired tool – comment, highlight, underline, or draw – then click anywhere on the document to add your annotation right away.
Final thoughts
Learning how to annotate PDF documents properly can make teamwork smoother and help everyone stay on the same page. Just a few well-placed hints in the form of texts, comments, or attached files can instantly shine a light on difficult concepts for your readers or teammates.
