A free tool for the

You can use PDF-XChange Viewer to highlight your pdf document. To use this PDF-XChange Viewer is very easy. You just need to click Tools on menu bar, choose Comment and Markup Tools, and then choose Highlight Text Tool. Now your pdf document is ready to be highlighted.
You also can manage your Highlight Text Tool such as you want to change the color or manage its Blend Mode. To manage your Highlight Text Tool, click Show Comment Styles Palette. The Blend Mode that you can choose such as: Normal, Multiply, Screen, Overlay, Darken, Lighten, Color Dodge, Color Burn, Hard Light, Soft Light, Difference, Exclusion, Hue, Saturation, Color, and Luminosity.
The others useful Comment and Markup Tools such as:
1. Sticky note. Note icon that you can choose for the sticky note such as: Checkmark, Circle, Comment, Cross, Help, Insert Text, Key, New Paragraph, Text Note, Paragraph, Right Arrow, Right Pointer, Star, Up Arrow, Up-Left Arrow.
2. Stamp Tool, such as APPROVED, AS IS, CONFIDENTIAL, DEPARTMENTAL, DRAFT, EXPERIMENTAL, EXPIRED, FINAL, FOR COMMENT, FOR PUBLIC RELEASE, NOT APPROVED, NOT FOR PUBLIC RELEASE, SOLD, and TOP SECRET.
3. File Attachment. Note icon that you can choose for the File Attachment such as: Graph, paperclip, Attchment, and tag.
Beside the above three, the others Comment and Markup Tools such as Typewriter Tool, text Box Tool, Callout tool, cross Out Text tool, Ynderline Text Tool, Arrow Tool, line Tool, Rectangle Tool, Oval Tool, Polygon Tool, Polygon Line Tool, Cloud Tool, Pencil Tool, Eraser Tool.
By using PDF-XChange Viewer, you also can add Rectangle Link and Quadrilateral Link on your pdf document.
You can download PDF-XChange Viewer from: http://www.pdfxviewer.com/