Adjusting Thickness of Multiple Horizontal Lines

Your sample pdf is showing 3 types of lines, the first two lines are actually rectangles, about 1pt. in size, most other lines are actual lines (strokes), the last 3 lines are type (fonts). One option would be to re-do the document in Word so all the lines are consistent, you would not need to re-do the form fields, as you can do a file> replace (Tools> Organize pages> Replace), using the updated pdf as the new file, that would preserve the original form fields. Another option would be to go to Tools> Print Production> Fix Hairlines (make them 1pt.). This would only effect the strokes, so the font lines would not change. There is also a preflight profile that works pretty much the same as Fix Hairlines. Another option would be to place the pdf into InDesign where you could replace all of the existing lines, delete the original lines in Acrobat, update the InDesign link, export to a new pdf.

I should also mention that different types of lines may appear to be different sizes, even if the sizes are the same, depending on pdf viewer and preferences.