Before copying you need to select text by turning on text selection mode using Select Text button on the document toolbar and selecting text to be copied with mouse. If you can select text (which is marked by green highlight), pressing Ctrl-C should copy the text to clipboard. Alternative methods for text copying are pressing Ctrl-Ins or right clickng your selection and selecting Copy Text.
If you cannot select text using mouse, check if the document has not been created with Disable Text Copying option available in Document Options window. This option prevents copying document text to clipboard.
Another reason for not being able to select text might be that your document contains no retrievable text. The text in your document might be represented as image and not as text and hence it cannot be copied. For example, this can happen for some PDF documents converted to Print2Flash documents with Adobe Reader. The fact is it may sometimes send text to printers not as text but as images. This issue is known to happen only with Adobe Reader and only for some documents. The fact is printing feature in Adobe Reader is oriented towards printing on real printers and getting a hard copy of documents. As it does not matter whether text is represented by text or by images when it is printed on paper, Adobe Reader seems not to be over-scrupulous to represent text as text when sending it to printers. We have not found a reliable solution for this Adobe Reader problem yet. If you obtained the PDF document from another document type, e.g. from MS Word, it is better to convert the original document rather than PDF. As we have said, this issue is known to happen only with Adobe Reader so direct conversion should resolve this problem.