If you have a problem copying the text from a PDF document, please check the following three things.
Check if you've switched to Select Text and Image tool
If the Hand tool is selected when you hover your mouse over the text, go to Home > Tools > Select > click Select Text and Image then try to copy.
Check if Content Copying is allowed or not:
Open the PDF document in Reader, go to File > Properties > Security, and review Document Restrictions Summary to see if Content Copying is allowed or not. If you are restricted from copying, contact the author of the PDF to change the security setting.
Check if the document is a scanned or image based file:
Open the PDF document in Reader, go to File > View > Text Viewer to see if there are text objects. If not, you could use OCR (Optical Character Recognition) feature in Foxit PhantomPDF Standard Edition/Business Edition to make the text recognizable and highlight-able. You could download Foxit PhantomPDF from here.
Tip: If the document is a scanned or image based file and you choose not to use the OCR feature in Foxit PhantomPDF, you can copy a section with the Snapshot tool.