Annotating a PDF, adding comments, highlights, sticky notes, underlines, and freehand drawings, is something people do regularly for work, study, and reviewing documents. Adobe Acrobat is the reference tool for this, but its subscription cost is hard to justify for occasional use. There are several genuinely capable free alternatives. Here is what works and where each option falls short.
macOS Preview: underrated and already installed
If you are on a Mac, Preview supports a surprisingly comprehensive annotation toolkit for free. Open any PDF in Preview and click the annotation toolbar (the pencil icon, or Markup Toolbar in View menu). You get:
- Text highlighting in multiple colours
- Underline and strikethrough
- Sticky note comments
- Freehand drawing with a pen tool
- Text boxes you can position anywhere on the page
- Shapes (rectangles, ovals, lines, arrows)
Annotations are saved in the PDF and are visible to recipients who open the file in any PDF viewer. For collaborative review, Preview is genuinely useful without any additional tools or accounts. The limitation is that it only runs on Mac.
Google Docs: free, browser-based, but limited
Google Docs can open a PDF and convert it for editing, but this converts the PDF to a Docs format, not an annotated PDF. If you need to leave comments for a collaborator within a PDF structure, Google Docs is not the right route.
Google Drive's own PDF viewer does allow basic annotation: you can highlight text by selecting it, and add a comment linked to the selection. These annotations are saved in Google Drive but only visible in the Drive viewer, not embedded in the PDF file itself. If you download the PDF, the annotations are not included.
Useful for quick comments within a collaborative Google Drive workflow, but not for producing an annotated PDF to send or share outside that environment.
Adobe Acrobat Reader (free): the best cross-platform annotator
Adobe Acrobat Reader is free and supports a good range of annotations: highlighting, sticky notes, underlines, strikethrough, text comments, and stamps. Annotations are embedded in the PDF and visible in any PDF viewer. For reliable cross-platform annotation that will display correctly for all recipients, Acrobat Reader is the most dependable free option.
The limitations of the free version are in editing (you cannot change existing content, only annotate) and in certain advanced features locked to the paid Acrobat Pro tier. For pure annotation, reviewing, commenting, marking up, the free Reader is sufficient.
Browser built-in viewers: limited but always available
Chrome, Firefox, and Edge all have built-in PDF viewers with minimal annotation support. Chrome's viewer allows basic text selection and the ability to save annotations, but the annotation toolkit is very limited compared to dedicated tools. Firefox's viewer is primarily a viewer with no annotation functionality. Edge has improved its PDF annotation, it supports ink drawing, highlighting, and typed notes, and is a practical free option on Windows 10 and Windows 11 for quick annotations.
Mobile annotation: iPhone and Android
On iPhone, the Files app allows basic PDF annotation directly: open a PDF, tap the pen icon in the top right, and use the Markup toolbar to highlight, draw, and add text. This is practical for quick annotations without a separate app. The Adobe Acrobat app (free on iOS) offers more options including comments, sticky notes, and shapes.
On Android, Adobe Acrobat Reader is the most capable free annotation app. Samsung devices include a PDF annotator in Samsung Notes and Samsung Files that is reasonable for basic use. Google Drive's built-in viewer allows highlighting within the Drive environment but not embedded in the exported PDF.
Adding text to any PDF position
For adding text at arbitrary positions on a PDF page, filling in flat forms, labelling figures, adding information not connected to existing text, PDFWhisk's sign and annotate tool lets you place text elements anywhere on any PDF page in your browser. This is useful for documents where you need to add typed content outside the margins, label diagrams, or complete forms that do not have interactive fields.
Saving and sharing annotated PDFs
Before sharing an annotated PDF, check that the annotations are embedded (not just overlaid in the viewer) by opening the downloaded file in a different PDF viewer. An annotation visible in Adobe Reader should also be visible in Chrome's PDF viewer if it is properly embedded. If the annotations disappear when you switch viewers, the file may have been saved without embedding the changes, check the save options in the tool you used.