PDFs and some image file types can be edited and annotated directly in Fieldguide's document viewer. Here are some of the available annotation tools that you can access from the toolbar:
Annotate: Highlight, underline, strike out, or use free text.
Shapes: Free hand highlight, free hand write, and add lines and shapes.
Insert: Insert stamps, signatures, and callout text boxes.
Edit: Use the snipping tool.
Fill and Sign: Insert stamps, signatures, free hand text, a cross (x), tic, dot, and date.
Use the 
panel button on the toolbar to open the page overview on the left. Click the 3-dots button under any page thumbnail to move, rotate, insert, delete, replace, or extract pages.
Note on file versions:
Edits and annotations made to PDFs will be saved as a new version of the file.
Edits and annotations made to image files will save to the original version of the file.