There are several tools to annotate PDF files. Some are non-free like Adobe Acrobat Writer and others, and some are free like Xournal on Linux. Xournal is a very powerful annotator, whereby, the original file is untouched, but a separate file of the annotations done on top of the original PDFs is maintained.
You can use annotations to fill up PDF files that contain forms like KYC, applications, instead of writing the information by hand into a printed copy of the form, or even proof-read PDF files and put your own markups into them.
An equivalent tool to Xournal is Xodo available for Android devices at https://play.google.com/store/apps/details?id=com.xodo.pdf.reader&hl=en on the Google Play Store. Xodo is extremely powerful and user-friendly. I find it is one of the best PDF annotation applications available for Android.