Hello there, elmo5676.
It sounds like you're trying to link to a weblink of an online PDF document in iBooks Author. The following link from the online iBooks Author Help Guide provides some great information on how to do that:
Add links and bookmarks - iBooks Author Help
You can create text or image links that your readers can tap or click to go to another place in the book, go to a webpage, or open a preaddressed email message.
As you create your book, you can quickly navigate to specific book locations by adding bookmarks. If you add links to your bookmarks, your readers can also use them in the completed book.
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Select the text or image you want to become a link.
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If the Link inspector isn’t open, click the Inspector button in the toolbar and click the Link inspector button .
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In the Link inspector, click Hyperlink.
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Select the “Enable as a hyperlink” checkbox.
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Click the Link To pop-up menu and choose an option:
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Webpage: Enter a URL.
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Email Message: Enter the email address and subject line.
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Bookmark: Click the Name pop-up menu and choose a bookmark.
If the Name pop-up menu is empty, add bookmarks as described below.
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Figure: Click a figure in the Name list. To view only specific types of figures (for example, all movies or galleries), click the Style pop-up menu and choose an option.
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Chapter or Section: Click the Chapter pop-up menu and choose an option.
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Page Number: Enter a page number.
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Kind Regards.