diff --git a/README.md b/README.md
index 3ca9b74..3a30f15 100644
--- a/README.md
+++ b/README.md
@@ -3,10 +3,10 @@ Minimum Viable Goals:
 - [x] Continuous Horizontal Scroll - unreasonably rare feature
 - [x] Presentation View - i.e. show on an external monitor - haven't seen another app do this yet!
 - [x] Immersive View - no system status bar etc.
+- [x] Autocrop margins
 - [ ] Open files the Android way - currently just a bundled test file
 - [ ] Delete+Reorder Pages - doesn't have to save the source file, but does need to save a change journal
 - [ ] Text Annotations - as above
-- [ ] Autocrop margins
 
 Stretch Goals:
 - [ ] Networked View:
diff --git a/app/src/main/java/com/lhw/pdf/PdfDocument.kt b/app/src/main/java/com/lhw/pdf/PdfDocument.kt
new file mode 100644
index 0000000..d0a776f
--- /dev/null
+++ b/app/src/main/java/com/lhw/pdf/PdfDocument.kt
@@ -0,0 +1,144 @@
+package com.lhw.pdf
+
+import android.graphics.Bitmap
+import android.graphics.Color
+import android.graphics.Matrix
+import android.graphics.Rect
+import android.graphics.RectF
+import android.graphics.pdf.PdfRenderer
+import android.os.ParcelFileDescriptor
+import androidx.core.graphics.times
+import java.io.File
+import kotlin.math.max
+import kotlin.math.min
+
+class PdfDocument(private val fileCached: File, private val autoCrop: Boolean = false) {
+    private val pfd = ParcelFileDescriptor.open(fileCached, ParcelFileDescriptor.MODE_READ_ONLY)
+    private val renderer = PdfRenderer(pfd)
+    val bitmapThumbnails = mutableMapOf<Int, Bitmap>()
+    val bitmapPagesMain = mutableMapOf<Int, Bitmap>()
+    val bitmapPagesPresentation = mutableMapOf<Int, Bitmap>()
+    private val pagePtWidths = mutableListOf<Int>()
+    private val pagePtHeights = mutableListOf<Int>()
+    private val pageAutoCropRects = mutableListOf<RectF>()
+    // Due to the way thumbnails render, the bottom right is more likely to be cut off
+    private val autoCropSafetyMarginUpper = 2
+    private val autoCropSafetyMarginLower = 16
+
+    init {
+        val numPages = renderer.pageCount
+        for (i in 0..<numPages) {
+            val page = renderer.openPage(i)
+            pagePtWidths.add(page.width)
+            pagePtHeights.add(page.height)
+            page.close()
+        }
+    }
+
+    private val rectFIdentity = RectF(0F, 0F, 1F, 1F)
+    private fun renderPage(index: Int, maxWidth: Int, maxHeight: Int, crop: Boolean = false): Bitmap {
+        val cropRect = if (!crop || pageAutoCropRects.size <= index) rectFIdentity else pageAutoCropRects[index]
+        val ptWidth = pagePtWidths[index] * cropRect.width()
+        val ptHeight = pagePtHeights[index] * cropRect.height()
+        val widthFromMaxHeight = (ptWidth * maxHeight/ptHeight).toInt()
+        val heightFromMaxWidth = (ptHeight * maxWidth/ptWidth).toInt()
+        val (width, height) = if (widthFromMaxHeight > maxWidth) Pair(maxWidth, heightFromMaxWidth) else Pair(widthFromMaxHeight, maxHeight)
+
+        val page = renderer.openPage(index)
+        println("Creating page $index bitmap with width $width and height $height from cropRect $cropRect")
+        val bitmap = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_8888)
+        val ptToPixel = height/ptHeight
+        val transform = Matrix()
+        val transposePtX = -cropRect.left * ptWidth
+        val transposePtY = -cropRect.top * ptHeight
+        transform.setValues(floatArrayOf(ptToPixel, 0F, transposePtX*ptToPixel, 0F, ptToPixel, transposePtY*ptToPixel, 0F, 0F, 1F))
+        page.render(bitmap, null, transform, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY)
+        page.close()
+        return bitmap
+    }
+
+    private fun renderPagesToMap(map: MutableMap<Int, Bitmap>, width: Int, height: Int, crop: Boolean = false) {
+        map.clear()
+        val numPages = renderer.pageCount
+        for (i in 0..<numPages) {
+            map[i] = renderPage(i, width, height, crop)
+        }
+    }
+
+    private fun isColorContent(c: Int): Boolean {
+        val color = Color.valueOf(c)
+        return (color.alpha() > 0) && (color.luminance() < 0.9F)
+    }
+    fun renderThumbnails(width: Int, height: Int) {
+        renderPagesToMap(bitmapThumbnails, width, height)
+        println("Thumbnails rendered")
+        if (autoCrop) {
+            println("Auto cropping from thumbnails")
+            val numPages = renderer.pageCount
+            pageAutoCropRects.clear()
+            for (i in 0..<numPages) {
+                println("Auto cropping page $i from thumbnail")
+                val bitmap = bitmapThumbnails[i]!!
+                val w = bitmap.width
+                val h = bitmap.height
+                val rect = Rect()
+                val rectF = RectF(rectFIdentity)
+                // Find first row
+                top@ for (y in 0..<h) {
+                    for (x in 0..<w) {
+                        val c = bitmap.getPixel(x, y)
+//                        println("Auto crop checking pixel $x $y == $c")
+                        if (isColorContent(c)) {
+                            rect.top = max(y-autoCropSafetyMarginUpper, 0)
+                            rectF.top = rect.top/h.toFloat()
+                            break@top
+                        }
+                    }
+                }
+                // Find last row
+                bottom@ for (y in (h-1) downTo 0) {
+                    for (x in 0..<w) {
+                        val c = bitmap.getPixel(x, y)
+                        if (isColorContent(c)) {
+                            rect.bottom = min(y+autoCropSafetyMarginLower, h-1)
+                            rectF.bottom = (rect.bottom+1)/h.toFloat()
+                            break@bottom
+                        }
+                    }
+                }
+                // Find first column
+                left@ for (x in 0..<w) {
+                    for (y in rect.top..rect.bottom) {
+                        val c = bitmap.getPixel(x, y)
+                        if (isColorContent(c)) {
+                            rect.left = max(x-autoCropSafetyMarginUpper, 0)
+                            rectF.left = rect.left/w.toFloat()
+                            break@left
+                        }
+                    }
+                }
+                // Find last column
+                right@ for (x in (w-1) downTo 0) {
+                    for (y in rect.top..rect.bottom) {
+                        val c = bitmap.getPixel(x, y)
+                        if (isColorContent(c)) {
+                            rect.right = min(x+autoCropSafetyMarginLower, w-1)
+                            rectF.right = (rect.right+1)/w.toFloat()
+                            break@right
+                        }
+                    }
+                }
+                println("Auto crop found margins of page $i at $rect, scales to $rectF")
+                pageAutoCropRects.add(rectF)
+            }
+        }
+    }
+
+    fun renderPagesMain(width: Int, height: Int) {
+        renderPagesToMap(bitmapPagesMain, width, height, true)
+    }
+
+    fun renderPagesPresentation(width: Int, height: Int) {
+        renderPagesToMap(bitmapPagesPresentation, width, height, true)
+    }
+}
\ No newline at end of file
diff --git a/gradle/libs.versions.toml b/gradle/libs.versions.toml
index 21ba284..c235899 100644
--- a/gradle/libs.versions.toml
+++ b/gradle/libs.versions.toml
@@ -3,15 +3,15 @@ agp = "8.5.2"
 kotlin = "1.9.0"
 coreKtx = "1.13.1"
 junit = "4.13.2"
-junitVersion = "1.1.5"
-espressoCore = "3.5.1"
+junitVersion = "1.2.1"
+espressoCore = "3.6.1"
 appcompat = "1.7.0"
-material = "1.10.0"
+material = "1.12.0"
 constraintlayout = "2.1.4"
-lifecycleLivedataKtx = "2.6.1"
-lifecycleViewmodelKtx = "2.6.1"
-navigationFragmentKtx = "2.6.0"
-navigationUiKtx = "2.6.0"
+lifecycleLivedataKtx = "2.8.4"
+lifecycleViewmodelKtx = "2.8.4"
+navigationFragmentKtx = "2.7.7"
+navigationUiKtx = "2.7.7"
 
 [libraries]
 androidx-core-ktx = { group = "androidx.core", name = "core-ktx", version.ref = "coreKtx" }