Resolved bad end stream checks that slowed stream object reading.

Author: mlaukala

src/PdfSharp/Pdf.IO/Lexer.cs

@@ -230,22 +230,49 @@ public byte[] ReadStream(int length)
             else
                 pos = _idxChar + 1;
 
-			// Verify stream length and resolve if bad
-			string post_stream = ReadRawString(pos + length, ("endstream").Length);
-			if (post_stream != "endstream")
+            // Producer: 
+            // Problem: Incorrect stream length
+            // Fix: Find the endstream keyword and measure the length
+            // https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PDF32000_2008.pdf 7.3.8
+
+            // Producer: 
+            // Problem: Not all pdf producers add a eol marker before endstream
+            // Fix: double check for endstream without the eol marker
+            // https://www.adobe.com/content/dam/acom/en/devnet/acrobat/pdfs/PDF32000_2008.pdf 7.3.8
+
+            // Verify stream length and resolve if bad
+            string nendstream = $"{'\n'}endstream";
+            string endstream = "endstream";
+
+			string postStream = ReadRawString(pos + length, nendstream.Length);
+            
+            bool bValid = postStream == nendstream ||
+                          postStream.StartsWith(endstream); // Not all pdf producers add a eol marker before endstream
+
+            if (!bValid)
 			{
 				// find the first endstream occurrence
 				// first check to see if it is within the specified stream length.
-				int endstream_idx = post_stream.IndexOf("endstream", StringComparison.Ordinal);
-				if (endstream_idx == -1)
+                int endStreamIdx = postStream.IndexOf(nendstream, StringComparison.Ordinal);
+                if (endStreamIdx == -1)
+                {
+                    endStreamIdx = postStream.IndexOf(endstream, StringComparison.Ordinal);
+                }
+
+                if (endStreamIdx == -1)
 				{
-					post_stream = ReadRawString(pos, _pdfLength - pos);
-					endstream_idx = post_stream.IndexOf("endstream", StringComparison.Ordinal);
-				}
+                    // TODO:: read in chunks
+					postStream = ReadRawString(pos, _pdfLength - pos);
+					endStreamIdx = postStream.IndexOf(nendstream, StringComparison.Ordinal);
+                    if (endStreamIdx == -1)
+                    {
+                        endStreamIdx = postStream.IndexOf(endstream, StringComparison.Ordinal);
+                    }
+                }
 
-				if (endstream_idx != -1)
+				if (endStreamIdx != -1)
 				{
-					length = endstream_idx;
+					length = endStreamIdx;
 				}
 			}