Is it possible to read a pdf file, like a user manual, with a cellphone?
Is there some kind of app?

There are several apps for iPhone that will read pdfs -- iFile Express, GoodReader, FileApp Pro, many others. Actually the three I mentioned will read many different document types.

What type of cell phone do you have?
Is it an Android?
You can do a search for the model of the phone and the words
pdf reader

Not sure what phone she has.

If she has Android, there is an Adobe Acrobat PDF Reader in the Google Play Store. I have it on my phone.