You may need to use the "Save Next Link As..." or "Save to Disk" functions of your Web browser to save the Lightning-Detector.pdf file.
If you have installed Acrobat Reader Plugin into your Netscape Navigator you can view the Log-Periodic Antennae Design Plans by just clicking this link otherwise, to download the PDF on Macintosh, hold down "option" key and click the link. A dialog will pop-up and allow you to download the file, where you want, to your hard drive.