Would you provide more information so that we can help you?
Which app do you use? Is it the mobile Reader app on an iPad/iPhone or Android tablet/phone or the desktop Reader/Acrobat app on a Windows or Mac computer?
What is the version of the app?
What is the version of the operating system on your device/computer?
By "EDIT", what kind of operations are you trying to perform? Filling out PDF forms, adding annotations (Sticky Notes, Highlight, Freehand Drawing, etc.), or something else?
Have you tried a couple of different PDF documents? Does the problem occur with all of your PDF documents or a particular PDF document?