If you feel like the text on your PDF files is too light to make sense of or you’re just generally looking for your PDFs to have a darker print, you can make the suitable changes on Adobe Acrobat.
Go to Edit > Preferences > General
Here, under the Accessibility tab, check what the Document color option says and switch it to Black
Go to File > Print and click on the Advanced button. Check the Print as Image box and try if it prints in a darker color