add doc comments, make point size internal until I can learn matrices, add test case for ascii 85 filter, add test case for document text

src/UglyToad.Pdf.Tests/Filters/Ascii85FilterTests.cs

@@ -41,7 +41,7 @@ O<DJ+*.@<*K0@<6L(Df-\0Ec5e;DffZ(EZee.Bl.9pF""AGXBPCsi + DGm >@3BB / F * &OCAfu2
 
             Assert.Equal("Man \0\0\0\0is d", text);
         }
-
+        
         [Fact]
         public void ZInMiddleOf5CharacterSequenceThrows()
         {
@@ -52,6 +52,16 @@ O<DJ+*.@<*K0@<6L(Df-\0Ec5e;DffZ(EZee.Bl.9pF""AGXBPCsi + DGm >@3BB / F * &OCAfu2
             Assert.Throws<InvalidOperationException>(action);
         }
 
+        [Fact]
+        public void SingleCharacterLastThrows()
+        {
+            var bytes = Encoding.ASCII.GetBytes("9jqo^B");
+
+            Action action = () => filter.Decode(bytes, new PdfDictionary(), 1);
+
+            Assert.Throws<ArgumentOutOfRangeException>(action);
+        }
+
         private const string PdfContent = @"1 0 obj
 << /Length 568 >>
 stream

src/UglyToad.Pdf.Tests/Integration/FontSizeTestFromGoogleChromeTests.cs

@@ -2,6 +2,7 @@
 {
     using System;
     using System.IO;
+    using System.Linq;
     using Xunit;
 
     public class FontSizeTestFromGoogleChromeTests
@@ -35,5 +36,18 @@
                 Assert.Equal(842, page.Height);
             }
         }
+
+        [Fact]
+        public void GetsCorrectPageTextIgnoringHiddenCharacters()
+        {
+            using (var document  = PdfDocument.Open(GetFilename()))
+            {
+                var page = document.GetPage(1);
+
+                var text = string.Join(string.Empty, page.Letters.Select(x => x.Value));
+
+                Assert.Equal("Hello, this is 16ptHello, this is 16px", text);
+            }
+        }
     }
 }

src/UglyToad.Pdf/Content/Letter.cs

@@ -9,6 +9,9 @@
         /// </summary>
         public string Value { get; }
 
+        /// <summary>
+        /// The lower-left position of the letter. Letters with descenders will extend below this point.
+        /// </summary>
         public PdfPoint Location { get; }
 
         /// <summary>
@@ -17,9 +20,9 @@
         public decimal Width { get; }
 
         /// <summary>
-        /// Size defined by the Tj operator prior to our possibly incorrect transformation.
+        /// Size as defined in the PDF file. This is not equivalent to font size in points but is relative to other font sizes on the page.
         /// </summary>
-        internal decimal FontSize { get; }
+        public decimal FontSize { get; }
 
         /// <summary>
         /// The name of the font.
@@ -27,10 +30,13 @@
         public string FontName { get; }
 
         /// <summary>
-        /// The size of the font in points.
+        /// The size of the font in points. This is not ready for public consumption as the calculation is incorrect.
         /// </summary>
-        public decimal PointSize { get; }
+        internal decimal PointSize { get; }
 
+        /// <summary>
+        /// Create a new letter to represent some text drawn by the Tj operator.
+        /// </summary>
         internal Letter(string value, PdfPoint location, decimal width, decimal fontSize, string fontName, decimal pointSize)
         {
             Value = value;

src/UglyToad.Pdf/Geometry/PdfPoint.cs

@@ -1,11 +1,28 @@
 namespace UglyToad.Pdf.Geometry
 {
+    /// <summary>
+    /// A point in a PDF file. 
+    /// </summary>
+    /// <remarks>
+    /// PDF coordinates are defined with the origin at the lower left (0, 0).
+    /// The Y-axis extends vertically upwards and the X-axis horizontally to the right.
+    /// Unless otherwise specified on a per-page basis, units in PDF space are equivalent to a typographic point (1/72 inch).
+    /// </remarks>
     public struct PdfPoint
     {
+        /// <summary>
+        /// The origin of the coordinates system.
+        /// </summary>
         public static PdfPoint Origin = new PdfPoint(0m, 0m);
 
+        /// <summary>
+        /// The X coordinate for this point. (Horizontal axis).
+        /// </summary>
         public decimal X { get; }
 
+        /// <summary>
+        /// The Y coordinate of this point. (Vertical axis).
+        /// </summary>
         public decimal Y { get; }
 
         public PdfPoint(decimal x, decimal y)