Question: I have an HP Envy Photo 6255 printer. Is there a way to modify the printer so it will accept only black ink cartridges? — Jan Hunt, Woodbury, Minn.

Answer: Yes, but there are some hidden costs. HP, like other printer manufacturers, operates on the old razor and razor blades business model: Printers are sold cheaply and ink is sold at high prices.

By some estimates, a lot of printers are sold at a loss just to enable the sale of profitable ink cartridges (see tinyurl.com/58usy9kh). This affects consumers in different ways:

• The kind of printer you have will allow you to remove the color cartridge and print only with the black cartridge (called “single cartridge mode.”)

That’s because your printer uses an ink-squirting mechanism, or “print head,” that’s contained inside each ink cartridge. This design frees you from having to buy expensive color ink cartridges that you don’t want.

• The other kind of printer can’t print anything unless it has both black and color ink cartridges present, and all contain at least some ink. HP says that’s because the print head is part of the printer, not the cartridge, and as a result both types of ink are needed to clear the ink pathways during automatic maintenance. That clearly adds to the cost of buying ink. (For a list of HP ink jet printers that require multiple cartridges, see tinyurl.com/4ryt4pfw).

But here’s the surprise. While your printer can get along on only a black ink cartridge, HP charges you for the privilege: Your ink cartridges cost more for the amount of ink they contain.

So, if you print frequently, your ink costs can be higher than they would be for a printer that requires multiple ink cartridges in order to print (see tinyurl.com/c2e64dhe).

Here’s some advice for readers who are considering buying a new printer: There appears to be a workaround that can lower ink costs. The trick is to buy an HP printer that requires both black and color print ink cartridges for printing.

Then return the printer to its factory settings (see tinyurl.com/dsknnvhw), which will remove the HP software that enforces the multiple ink cartridge requirement. You will be able to use only a black cartridge without paying extra for ink.

Question: I want to make a minor edit on a PDF file, but I can’t get the file to save the change once I make it. What should I do? — Kenneth Janda, Roseville, Minn.

Answer: The problem could be caused by using the wrong software or incorrect program settings. Or, it could be caused by restrictions that are built into that particular PDF file.

First make sure you have the right program. The widely used free Adobe Acrobat Reader DC can fill out a PDF form and save it, but it can’t edit text that’s already in a PDF.

For that you need the Adobe Acrobat program (PC version $300 one-time purchase, online version $156 a year) that can create or edit PDF files. Alternatively, you can edit PDF files with some other programs, including Microsoft Word, Google Docs or a PDF editor program (see tinyurl.com/hzw5f85c).

If you are using Adobe Acrobat and can’t edit a PDF document, you might need to set the program as your computer’s “default PDF viewer” (see tinyurl.com/3m7scy9c).

You should check to see if the file is “restricted for editing” (see tinyurl.com/3wz8jkf6).