how do I resize an image once it is pasted on the pdf?

Using the Edit Text & Images tool under Tools - Content Editing.

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Which version of Acrobat do you have? With Acrobat XI you would select Tools>Content Editing>Edit Text&Images, then click on the image. This will select the image, and it will also show you resize handles around the image. When you grab one of these handles with your mouse, you can resize the image:

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If you have Adobe Acrobat XI go to menu View -> Tools -> Content Editing, choose Edit Text & Images, select the image and resize it.

If you have Adobe Acrobat X there is no "Edit Text & Images" tool, but you can go to menu View -> Tools -> Content and use the "Edit Object" tool to select and resize an image. In Acrobat XI Pro this tool is under "Print Production" panel.