Sumatra PDF is a PDF, ePub, MOBI, CHM, XPS, DjVu, CBZ, CBR reader for Windows

How to edit a document in Sumatra PDF?

How to edit a document in Sumatra PDF

Sumatra is a reader, not an editor, so we don’t have editing functionality.

[Related discussion at How to fill PDF forms offline?]

Since this seems to be the go to post with the highest hits on the topic

SumatraPDF does not have editor functions
However may modify PDF (and some other files) via additional functions

To be clear an editor changes the original data
Annotations can be appended to the file without editing the source

So additions fall under the common term Annotation

Form Filling is one type of adding text see above link: How to fill PDF forms offline?.

Highlighting Areas is another form of adding color, say to highlight some text

Embedding external files or even associating external notes are other types of addition

See the first earlier discussions at

Signatures / DRM are IMHO a waste of lifes resources for the oil seller, author, editor, cracker and reader, they are in an addition class of their own, and frequently targeted / often broken. Added to the file to say the original was at one time sealed and is not now changed ! the first breach is the reader has to unpack it to see it ?
Topic at How to add signature into Sumatra PDF?