Ok, so you are asking for build-in Viewer. This request is being tracked already:
Issue 138 – Internal viewer
You can vote for it there.

For text files, EDIT can change the file contents and the cursor moves inside line by line and column by column, which makes navigation slower/laggy. VIEW will not allow any change and the cursor (arrow) keys can quickly move/scroll the content (together with page up/down, home/end, mouse wheel etc.). Moreover, since you cannot type, many keyboard keys can have special functions, e.g., W key quickly turns wrapping on/off, other keys (1, 2, 3, 4) can quickly change value rendering (Text, Binary, Hexadecimal, Decimal ...). That is superefficient when looking for some information in the ASCII file.

The truth is, Total Commander or other clones have their own built-in viewer with these functionalities.

VIEWing a bitmap or PDF would simply show the image of PDF in external viewer such as IrfanView or SumatraPDF. Yes, this can be accomplished now with F4 but F4 is meant for editing (not viewing). For editing I would prefer to use Microsoft Paint (just kidding) or Acrobat for these files.

Thanks
Robert

Are you asking for another set of associated applications for viewing? Or how would the the View different from Edit?

It seems the F3 key is not used. Unfortunately, it is not possible to create such custom shortcut for a custom command, right?

Can you please add this key for a quick viewing of files (i.e. not editing) similar to Total Commander? This is super useful for ASCII text files and bitmaps. A custom viewer app could be optionally set up. So then F3 views the file and F4 edits the file...

Pretty please with sugar on top.