[Feature Request]: Add a freehand pen/drawing tool for PDF annotation

Feature Description

I would like to request a freehand pen tool for annotating PDFs directly inside the PDF viewer.

The user should be able to select a Pen tool and draw directly on the PDF using:

• Mouse
• Trackpad
• Stylus
• Touchscreen where supported

The drawing should follow the pointer/stylus smoothly and appear as an annotation on the page.

Basic controls could include:

• Pen color
• Pen thickness
• Opacity
• Eraser
• Undo/redo
• Clear selected drawing

Why is this feature valuable?

A freehand pen would make Stirling-PDF much more useful for education and document review.

For example, a teacher could open a PDF worksheet or textbook and use a mouse or stylus to:

• Circle important content
• Underline something manually
• Write short notes
• Draw arrows
• Mark an exercise
• Explain something directly on the PDF

This is especially useful when the application is being used in fullscreen/presentation mode.

Suggested Implementation

The pen could be implemented as a PDF annotation layer rather than immediately modifying the original PDF content.

The annotation layer should remain editable until the user saves/exports the document.

A simple toolbar could provide:

Pen | Highlighter | Eraser | Undo | Redo

with controls for color and stroke width.

Additional Information

It would be especially useful if the freehand annotation worked consistently both in normal viewer mode and fullscreen/reading mode.

Example Files

No response

No Duplicate of the Feature

• I have verified that there are no existing features requests similar to my request.