how to change work area for several pages

@pierret18811376 

Is this what you are trying to achieve?

• from 134,96 mm to 135 mm
• from 214,97 mm to 215 mm

Instead of 300 ppi (pixels per inch), the bottom left corner says 300 ppp. What does the third "p" stand for?

I live in the one part of the world that does not use metrics, but Photoshop cannot give you an image size that includes a partial pixel and will do the kind of rounding you are seeing when faced with an impossible situation.

Jane

@pierret18811376 

How did the scripts work for you?

This version uses content aware fill (therefore, it's slower) – in an attempt to seamlessly extend the variable background colour:

/*
Resize Canvas For All Open Docs.jsx
Stephen Marsh, v2.0 - 19th February 2022
https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-to-change-work-area-for-several-pages/td-p/12761765
*/

#target photoshop

// CHECK FOR OPEN DOCS
if (documents.length) {

    // SET THE CURRENT DOC AS ORIGINAL DOC
    var actDoc = activeDocument;

    // LOOP OVER OPEN FILES
    for (var i = 0; i < documents.length; i++) {
        activeDocument = documents[i];
        if (activeDocument.activeLayer.isBackgroundLayer) {
            activeDocument.activeLayer.name = 'Background';
            extendCanvas();
        } else {
            extendCanvas();
        }

    }

    // RETURN TO THE ORIGINAL DOC
    activeDocument = actDoc;

    // END OF SCRIPT NOTIFICATION
    app.beep();

    // ALERT IF NO DOCS OPEN
} else {
    alert('You must have a document open!');
}

// FUNCTIONS

function extendCanvas() {
    // Content aware fill requires a certain amount of blank space to work without erroring
    activeDocument.resizeCanvas(UnitValue(137, "mm"), UnitValue(217, "mm"), AnchorPosition.MIDDLECENTER);
    loadLayerTrans();
    activeDocument.selection.invert();
    contentAwareFill(true, false, false, false, 100);
    activeDocument.selection.deselect();
    // Final required size
    activeDocument.resizeCanvas(UnitValue(135, "mm"), UnitValue(215, "mm"), AnchorPosition.MIDDLECENTER);
    activeDocument.flatten();
}

function loadLayerTrans() {
	var s2t = function (s) {
		return app.stringIDToTypeID(s);
	};
	var descriptor = new ActionDescriptor();
	var reference = new ActionReference();
	var reference2 = new ActionReference();
	reference.putProperty(s2t("channel"), s2t("selection"));
	descriptor.putReference(s2t("null"), reference);
	reference2.putEnumerated(s2t("channel"), s2t("channel"), s2t("transparencyEnum"));
	descriptor.putReference(s2t("to"), reference2);
	executeAction(s2t("set"), descriptor, DialogModes.NO);
}

function contentAwareFill(contentAwareColorAdaptationFill, contentAwareRotateFill, contentAwareScaleFill, contentAwareMirrorFill, opacity) {
	var s2t = function (s) {
		return app.stringIDToTypeID(s);
	};
	var descriptor = new ActionDescriptor();
	descriptor.putEnumerated(s2t("using"), s2t("fillContents"), s2t("contentAware"));
	descriptor.putBoolean(s2t("contentAwareColorAdaptationFill"), contentAwareColorAdaptationFill);
	descriptor.putBoolean(s2t("contentAwareRotateFill"), contentAwareRotateFill);
	descriptor.putBoolean(s2t("contentAwareScaleFill"), contentAwareScaleFill);
	descriptor.putBoolean(s2t("contentAwareMirrorFill"), contentAwareMirrorFill);
	descriptor.putUnitDouble(s2t("opacity"), s2t("percentUnit"), opacity);
	descriptor.putEnumerated(s2t("mode"), s2t("blendMode"), s2t("normal"));
	executeAction(s2t("fill"), descriptor, DialogModes.NO);
}

These images appear to be scanned, with discolored backgrounds. The following script resets the background colour to pure white, which may or not exactly match the scan on all edges. 

/*
Resize Canvas For All Open Docs.jsx
Stephen Marsh, v1.1 - 19th February 2022
https://community.adobe.com/t5/photoshop-ecosystem-discussions/how-to-change-work-area-for-several-pages/td-p/12761765
*/

#target photoshop

// CHECK FOR OPEN DOCS
if (app.documents.length) {

    // SET THE CURRENT DOC AS ORIGINAL DOC
    var actDoc = activeDocument;

    resetSwatches();

    // LOOP OVER OPEN FILES
    for (var i = 0; i < app.documents.length; i++) {
        activeDocument = app.documents[i];
        activeDocument.resizeCanvas(UnitValue(135,"mm"), UnitValue(215,"mm"), AnchorPosition.MIDDLECENTER);
    }

    // RETURN TO THE ORIGINAL DOC
    activeDocument = actDoc;

    // END OF SCRIPT NOTIFICATION
    app.beep();

// ALERT IF NO DOCS OPEN
} else {
    alert('You must have a document open!');
}

function resetSwatches() {
	var s2t = function (s) {
		return app.stringIDToTypeID(s);
	};
	var descriptor = new ActionDescriptor();
	var reference = new ActionReference();
	reference.putProperty( s2t( "color" ), s2t( "colors" ));
	descriptor.putReference( s2t( "null" ), reference );
	executeAction( s2t( "reset" ), descriptor, DialogModes.NO );
}

1. Copy the JavaScript code text to the clipboard
2. Open a new blank file in a plain-text editor (not word-processor)
3. Paste the code in
4. Save the text file with your desired filename with a .txt filename extension
5. Rename the file extension from .txt to .jsx
6. Install or browse to the .jsx file to run

NOTE: Altering page box dimensions in Acrobat Pro is considered best practice. Using a plug-in such as Enfocus PitStop Pro can also extend the background colour to the images, even if the content is raster.