VintaSoft Web PDF Document Editor has "Editor" tab that contains UI for editing text content on PDF page.
UI allows to do the following operations:

• Change font family and font size for selected text
• Change style (bold, italic, underline, strikethrough) for selected text
• Change alignment (left, center, right, justify) for selected text
• Change color for selected text
• Undo/redo operations with text
• Add new text block to a PDF page
• Copy/paste text on PDF page
• Delete text from PDF page
• Save changes in text content on PDF page

VintaSoft Web PDF Document Editor edits text content on PDF page using WebPdfContentEditorToolJS visual tool. If you want programmatically edit text content in VintaSoft Web PDF Document Editor, please see information about functionality of WebPdfContentEditorToolJS visual tool.

Here is the screenshot of web PDF document editor with opened "Editor" tab: