Tesseract

See here for example

Get help with: 

tesseract.exe  --help-extra

.txt is automatically appended: 

tesseract.exe -l eng image.png textfile

Do a whole folder of images: 

for %i in (*.png) do tesseract.exe -l eng "%~i" "%~ni"

PDF

To OCR a PDF, first extract the images with Ghostscript: 

gswin32c -q -dBATCH -dNOPAUSE -sDEVICE=jpeg -dFirstPage=1 -dLastPage=4 -sOutputFile=imagefile_%d.jpg -r300 pdffile.pdf
for %i in (*.jpg) do tesseract.exe -l eng "%~i" "%~ni"