It seems to be a limitation of the PDF annotation tool itself - in many programs, the "freehand" automatically uses sharp (butt/miter) ends and does not allow you to change them.
Try checking the Pen/Brush settings: sometimes there are options like "line smoothing", "pressure" or "style" that soften the edges a bit. If not, it is worth looking at alternatives - for example, Adobe Acrobat Reader or Foxit PDF Editor sometimes give you more control over the appearance of the lines.
It seems to be a limitation of the PDF annotation tool itself - in many programs, the "freehand" automatically uses sharp (butt/miter) ends and does not allow you to change them.
Try checking the Pen/Brush settings: sometimes there are options like "line smoothing", "pressure" or "style" that soften the edges a bit. If not, it is worth looking at alternatives - for example, Adobe Acrobat Reader or Foxit PDF Editor sometimes give you more control over the appearance of the lines.