You can choose the type of file you want for the scanner's output. For photographs I'd always choose jpeg (.jpg). It's the compressed format which all digital cameras use by default.
For documents PDF might be a suitable format unless, say, you wanted to embed the image into another document. (For example, you might be writing your memoirs and want to write "Here's what my very first school report looked like", with a picture of it appended). Under those circumstances, jpeg would again be your best choice.
Note that, while most scanners (including those which come as part of an all-in-one device) have an automatic 'Scan' button on them, you'll get far more control of your scans if you use software on your computer to take control of your scanner. (For example, you'll be able to crop that old newspaper report of your success in the Cub Scout sports day to remove the corset advert which appeared alongside it. You'll also be able to get rid of the yellowing of the newspaper). Most scanners come with suitable software or you can use almost any image-editing program and go to File > Acquire to import a scan into it.