After conversion, if you can not edit text, chances are that the text was already stored as graphics in the PDF original. It was probably converted to curves or plotted as pen strokes when the PDF file was created. It is no longer text, just vector curves that look like text. This often happens for example with PDF drawings are created from CAD, EDA or GIS applications as well as with print advertisements from a DTP-package to ensure font display accurately. Text is often converted to curves to ensure accurate print results and to protect against font incompatibilities. Once the text has been turned into curves, there is no way for pdf2picture to retrieve it as real text. If the text is still searchable in the PDF file, we can produce it as editable text in the conversion output.
Tip: To see if the text is searchable and convertable or not, open your PDF file in Acrobat Reader and try to “Select” some text. If you can mark it, you can convert it.