recover from invalid acro-form references

we add a try/catch to the direct object finder's tryget method so it returns false rather than throwing.

if we have an acro-form reference in the catalog but no corresponding object in the document we instead scan all objects in the document to find form fields and reconstruct the acro-form dictionary.

src/UglyToad.PdfPig/AcroForms/AcroFormFactory.cs

@@ -5,6 +5,7 @@
     using System.Linq;
     using Content;
     using Core;
+    using CrossReference;
     using Exceptions;
     using Fields;
     using Filters;
@@ -30,11 +31,13 @@
 
         private readonly IPdfTokenScanner tokenScanner;
         private readonly IFilterProvider filterProvider;
+        private readonly CrossReferenceTable crossReferenceTable;
 
-        public AcroFormFactory(IPdfTokenScanner tokenScanner, IFilterProvider filterProvider)
+        public AcroFormFactory(IPdfTokenScanner tokenScanner, IFilterProvider filterProvider, CrossReferenceTable crossReferenceTable)
         {
             this.tokenScanner = tokenScanner ?? throw new ArgumentNullException(nameof(tokenScanner));
             this.filterProvider = filterProvider ?? throw new ArgumentNullException(nameof(filterProvider));
+            this.crossReferenceTable = crossReferenceTable ?? throw new ArgumentNullException(nameof(crossReferenceTable));
         }
 
         /// <summary>
@@ -44,10 +47,40 @@
         [CanBeNull]
         public AcroForm GetAcroForm(Catalog catalog)
         {
-            if (!catalog.CatalogDictionary.TryGet(NameToken.AcroForm, out var acroRawToken) || !DirectObjectFinder.TryGet(acroRawToken, tokenScanner, out DictionaryToken acroDictionary))
+            if (!catalog.CatalogDictionary.TryGet(NameToken.AcroForm, out var acroRawToken) )
             {
                 return null;
             }
+
+            if (!DirectObjectFinder.TryGet(acroRawToken, tokenScanner, out DictionaryToken acroDictionary))
+            {
+                var fieldsRefs = new List<IndirectReferenceToken>();
+
+                // Invalid reference, try constructing the form from a Brute Force scan.
+                foreach (var reference in crossReferenceTable.ObjectOffsets.Keys)
+                {
+                    var referenceToken = new IndirectReferenceToken(reference);
+                    if (!DirectObjectFinder.TryGet(referenceToken, tokenScanner, out DictionaryToken dict))
+                    {
+                        continue;
+                    }
+
+                    if (dict.TryGet(NameToken.Kids, tokenScanner, out ArrayToken _) && dict.TryGet(NameToken.T, tokenScanner, out StringToken _))
+                    {
+                        fieldsRefs.Add(referenceToken);
+                    }
+                }
+
+                if (fieldsRefs.Count == 0)
+                {
+                    return null;
+                }
+
+                acroDictionary = new DictionaryToken(new Dictionary<NameToken, IToken>
+                {
+                    { NameToken.Fields, new ArrayToken(fieldsRefs) }
+                });
+            }
             
             var signatureFlags = (SignatureFlags)0;
             if (acroDictionary.TryGetOptionalTokenDirect(NameToken.SigFlags, tokenScanner, out NumericToken signatureToken))

src/UglyToad.PdfPig/Parser/Parts/DirectObjectFinder.cs

@@ -20,17 +20,24 @@
                 return false;
             }
 
-            var temp = scanner.Get(reference.Data);
-
-            if (temp.Data is T tTemp)
+            try
             {
-                tokenResult = tTemp;
-                return true;
+                var temp = scanner.Get(reference.Data);
+
+                if (temp.Data is T tTemp)
+                {
+                    tokenResult = tTemp;
+                    return true;
+                }
+
+                if (temp.Data is IndirectReferenceToken nestedReferenceToken)
+                {
+                    return TryGet(nestedReferenceToken, scanner, out tokenResult);
+                }
             }
-
-            if (temp.Data is IndirectReferenceToken nestedReferenceToken)
+            catch
             {
-                return TryGet(nestedReferenceToken, scanner, out tokenResult);
+                return false;
             }
 
             return false;

src/UglyToad.PdfPig/Parser/PdfDocumentFactory.cs

@@ -148,7 +148,7 @@
 
             var caching = new ParsingCachingProviders(bruteForceSearcher, resourceContainer);
 
-            var acroFormFactory = new AcroFormFactory(pdfScanner, filterProvider);
+            var acroFormFactory = new AcroFormFactory(pdfScanner, filterProvider, crossReferenceTable);
             var bookmarksProvider = new BookmarksProvider(log, pdfScanner, isLenientParsing);
             
             return new PdfDocument(log, inputBytes, version, crossReferenceTable, isLenientParsing, caching, pageFactory, catalog, information,