Firefox can't do that as far as I know, so maybe best is to use an external PDF Viewer. Evince can do this via the "Ctrl+I" command.

• https://support.mozilla.org/en-US/kb/disable-built-pdf-viewer-and-use-another-viewer

I know that Firefox can't do that. My question is why? How can it be changed? Where should I put my hands on to hardcode it?

My screenshot shows exactly the solution you are talking about. So, for what concerns me, that's not a solution but the problem per se.

Every open source reader that I am using other than the Firefox embedded reader, implemented this option years ago. Maybe, it was time Firefox added it as well.

This isn't straightforward as reversing all colors can images look weird, but this might be necessary to some background images (colors).

See also this issue (some others are duped to this one): Unable to change the text and background colors. · Issue #2071

• https://github.com/mozilla/pdf.js/issues/2071

Hi ninoCan, Firefox treats the PDF Viewer as a part of the browser, which restricts what add-ons can do -- basically, they can't touch the document.

There is a way to apply an invert filter to the PDF viewer using a custom style rule in a userContent.css file. This is a bit of an all-or-nothing setting, so in the rare case of a light-on-dark PDF, it will be inverted to dark-on-light. To counteract that, you would need to inject an overriding style rule.

So if you have some time to experiment, like 15 minutes, this is how you would do it:

Set up Invert Filter in userContent.css

This assumes you do not already have a userContent.css file. If you do already have a working userContent.css file, you just need to add the rule under (A) to your file.

(A) Select and copy the following style rule code

/*** Invert PDF Viewer ***/
#outerContainer > #mainContainer > #viewerContainer > #viewer > .page {
  filter: invert(100%);
}

(B) Generate and download a userContent.css file

Open the following page and paste the above rules into the editor, replacing the sample rule:

https://www.userchrome.org/download-userchrome-css.html

Then change the file name selection to userContent.css and click "Generate CSS File". Save the userContent.css file to your computer. (See first attached screenshot)

Use the downloads list on the toolbar to open the downloads folder directly to the new userContent.css file. (See second attached screenshot)

Minimize (don't close) that File Explorer (Mac: Finder; Linux: ?) window for later reference.

(C) Create a new chrome folder in your profile folder

The following article has the detailed steps for that (#1, #2, and I recommend #3)

https://www.userchrome.org/how-create-userchrome-css.html

I have videos for Windows and Mac in case the text is not clear.

(D) Move the userContent.css file you generated in Step B into the chrome folder you created in Step C

You could copy/paste, drag/drop, etc.

(E) Set Firefox to look for userContent.css at startup -- see step #6 in the above article.

The next time you exit Firefox and start it up again, it should discover that file and apply the rules to the PDF viewer.

Success?

Set Up Bookmarklet to Undo (Override) the Invert Filter

(A) Select and copy the following script code

javascript:r='#viewerContainer > #viewer > .page {filter: unset !important;}'; s=document.createElement('style'); s.appendChild(document.createTextNode(r)); document.body.appendChild(s); void 0;

(B) Create the bookmarklet

• Right-click the Bookmarks Menu or Bookmarks Toolbar near where you want to add the bookmarklet
• Click New Bookmark
• Paste the script code into the Location field
• Type a name in the Name field, like UnvertPDF
• Click the Add button

When you want to override the invert filter, click the bookmarklet button.

Here is a bookmarklet that works as a toggle:

javascript:/*PDF - toggle filter invert*/(function(){var L='style_PDFI',S='#viewerContainer>#viewer.pdfViewer>.page{filter: invert(100%)}',SS,E=document.querySelector('style[id="'+L+'"]');if(E){SS=E.disabled;E.disabled=(SS==true)?false:true}else{SS=document.createElement('style');SS.setAttribute('type','text/css');SS.id=L;SS.innerHTML=S;document.querySelector('head').appendChild(SS);}})()

It looks better if I select all the text as that makes the text color yellow.

You can modify the bookmarklet to enable/disable the above posted style to override the style rule added via userContent.css.

javascript:/*PDF - toggle filter unset*/(function(){var L='style_PDFU',S='#viewerContainer>#viewer.pdfViewer>.page{filter:unset!important}',SS,E=document.querySelector('style[id="'+L+'"]');if(E){SS=E.disabled;E.disabled=(SS==true)?false:true}else{SS=document.createElement('style');SS.setAttribute('type','text/css');SS.id=L;SS.innerHTML=S;document.querySelector('head').appendChild(SS);}})()