select whole block of sentences

Thank you for the screenshot, I was able to see that you're using the older (better) UI. 

Be aware that not all PDFs can be edited. They have have been "Protected," they may have been OCRed from an image, there are a number of reasons that prevent editing. Just be aware.

First, be in the "Edit PDF" Toolset (green arrow), then click on the "Add Text" tool (blue arrow). Now you can select text and edit words. 

HOWEVER, please remember that Acrobat is NOT a word processor or a text layout application. If you try to do extensive editing, you will see the problem.

If you need to do extensive editing, the best thing to do is to get the original document and work on it in the original application. The 2nd best option is to export the document to Word, make the changes you want (and repair any damage the export did to the document), and the revert the document back to a PDF.