diff --git a/core/fpdfdoc/BUILD.gn b/core/fpdfdoc/BUILD.gn
index bb5c421..c917a5c 100644
--- a/core/fpdfdoc/BUILD.gn
+++ b/core/fpdfdoc/BUILD.gn
@@ -77,6 +77,7 @@
     "cpvt_wordrange.h",
     "ipvt_fontmap.h",
   ]
+  cflags = []
   configs += [ "../../:pdfium_strict_config" ]
   deps = [
     "../../constants",
@@ -88,6 +89,9 @@
     "../fxge",
   ]
   visibility = [ "../../*" ]
+  if (is_clang) {
+    cflags += [ "-Wshorten-64-to-32" ]
+  }
 }
 
 pdfium_unittest_source_set("unittests") {
diff --git a/core/fpdfdoc/cpdf_bookmark.cpp b/core/fpdfdoc/cpdf_bookmark.cpp
index 236b5ec..954831b 100644
--- a/core/fpdfdoc/cpdf_bookmark.cpp
+++ b/core/fpdfdoc/cpdf_bookmark.cpp
@@ -30,12 +30,12 @@
     return WideString();
 
   WideString title = pString->GetUnicodeText();
-  int len = title.GetLength();
+  size_t len = title.GetLength();
   if (!len)
     return WideString();
 
   std::vector<wchar_t, FxAllocAllocator<wchar_t>> buf(len);
-  for (int i = 0; i < len; i++) {
+  for (size_t i = 0; i < len; i++) {
     wchar_t w = title[i];
     buf[i] = w > 0x20 ? w : 0x20;
   }
diff --git a/core/fpdfdoc/cpdf_dest.cpp b/core/fpdfdoc/cpdf_dest.cpp
index 016df00..f3b1152 100644
--- a/core/fpdfdoc/cpdf_dest.cpp
+++ b/core/fpdfdoc/cpdf_dest.cpp
@@ -127,15 +127,15 @@
   return true;
 }
 
-unsigned long CPDF_Dest::GetNumParams() const {
+size_t CPDF_Dest::GetNumParams() const {
   if (!m_pArray || m_pArray->size() < 2)
     return 0;
 
-  unsigned long maxParamsForFitType = kZoomModeMaxParamCount[GetZoomMode()];
-  unsigned long numParamsInArray = m_pArray->size() - 2;
+  size_t maxParamsForFitType = kZoomModeMaxParamCount[GetZoomMode()];
+  size_t numParamsInArray = m_pArray->size() - 2;
   return std::min(maxParamsForFitType, numParamsInArray);
 }
 
-float CPDF_Dest::GetParam(int index) const {
+float CPDF_Dest::GetParam(size_t index) const {
   return m_pArray ? m_pArray->GetNumberAt(2 + index) : 0;
 }
diff --git a/core/fpdfdoc/cpdf_dest.h b/core/fpdfdoc/cpdf_dest.h
index 0118bb4..98f51d0 100644
--- a/core/fpdfdoc/cpdf_dest.h
+++ b/core/fpdfdoc/cpdf_dest.h
@@ -28,8 +28,8 @@
   // Returns the zoom mode, as one of the PDFDEST_VIEW_* values in fpdf_doc.h.
   int GetZoomMode() const;
 
-  unsigned long GetNumParams() const;
-  float GetParam(int index) const;
+  size_t GetNumParams() const;
+  float GetParam(size_t index) const;
 
   bool GetXYZ(bool* pHasX,
               bool* pHasY,
diff --git a/core/fpdfdoc/cpdf_formfield.cpp b/core/fpdfdoc/cpdf_formfield.cpp
index 61f64bc..456acea 100644
--- a/core/fpdfdoc/cpdf_formfield.cpp
+++ b/core/fpdfdoc/cpdf_formfield.cpp
@@ -232,7 +232,10 @@
 
   const auto& controls = GetControls();
   auto it = std::find(controls.begin(), controls.end(), pControl);
-  return it != controls.end() ? it - controls.begin() : -1;
+  if (it == controls.end())
+    return -1;
+
+  return pdfium::base::checked_cast<int>(it - controls.begin());
 }
 
 FormFieldType CPDF_FormField::GetFieldType() const {
@@ -422,7 +425,7 @@
   if (pValue->IsString() || pValue->IsNumber())
     return pValue->GetString().IsEmpty() ? 0 : 1;
   const CPDF_Array* pArray = pValue->AsArray();
-  return pArray ? pArray->size() : 0;
+  return pArray ? fxcrt::CollectionSize<int>(*pArray) : 0;
 }
 
 int CPDF_FormField::GetSelectedIndex(int index) const {
@@ -555,7 +558,7 @@
 
 int CPDF_FormField::CountOptions() const {
   const CPDF_Array* pArray = ToArray(GetFieldAttr(m_pDict.Get(), "Opt"));
-  return pArray ? pArray->size() : 0;
+  return pArray ? fxcrt::CollectionSize<int>(*pArray) : 0;
 }
 
 WideString CPDF_FormField::GetOptionText(int index, int sub_index) const {
@@ -687,18 +690,20 @@
 
 int CPDF_FormField::CountSelectedOptions() const {
   const CPDF_Array* pArray = ToArray(GetSelectedIndicesObject());
-  return pArray ? pArray->size() : 0;
+  return pArray ? fxcrt::CollectionSize<int>(*pArray) : 0;
 }
 
 int CPDF_FormField::GetSelectedOptionIndex(int index) const {
+  if (index < 0)
+    return 0;
+
   const CPDF_Array* pArray = ToArray(GetSelectedIndicesObject());
   if (!pArray)
     return -1;
 
-  int iCount = pArray->size();
-  if (iCount < 0 || index >= iCount)
-    return -1;
-  return pArray->GetIntegerAt(index);
+  return index < fxcrt::CollectionSize<int>(*pArray)
+             ? pArray->GetIntegerAt(index)
+             : -1;
 }
 
 bool CPDF_FormField::IsSelectedOption(const WideString& wsOptValue) const {
diff --git a/core/fpdfdoc/cpdf_interactiveform.cpp b/core/fpdfdoc/cpdf_interactiveform.cpp
index c60468b..00d5aef 100644
--- a/core/fpdfdoc/cpdf_interactiveform.cpp
+++ b/core/fpdfdoc/cpdf_interactiveform.cpp
@@ -673,7 +673,7 @@
     return 0;
 
   CPDF_Array* pArray = m_pFormDict->GetArrayFor("CO");
-  return pArray ? pArray->size() : 0;
+  return pArray ? fxcrt::CollectionSize<int>(*pArray) : 0;
 }
 
 CPDF_FormField* CPDF_InteractiveForm::GetFieldInCalculationOrder(int index) {
diff --git a/core/fpdfdoc/cpdf_linklist.cpp b/core/fpdfdoc/cpdf_linklist.cpp
index 0a4c791..0acd1fa 100644
--- a/core/fpdfdoc/cpdf_linklist.cpp
+++ b/core/fpdfdoc/cpdf_linklist.cpp
@@ -33,7 +33,7 @@
       continue;
 
     if (z_order)
-      *z_order = annot_index;
+      *z_order = pdfium::base::checked_cast<int32_t>(annot_index);
     return link;
   }
   return CPDF_Link();
diff --git a/core/fpdfdoc/cpdf_nametree.cpp b/core/fpdfdoc/cpdf_nametree.cpp
index 56d58e0..dfd4e61 100644
--- a/core/fpdfdoc/cpdf_nametree.cpp
+++ b/core/fpdfdoc/cpdf_nametree.cpp
@@ -193,8 +193,7 @@
       if (ppFind)
         *ppFind = pNames;
       if (pFindIndex)
-        *pFindIndex = pNames->size() / 2 - 1;
-
+        *pFindIndex = fxcrt::CollectionSize<int32_t>(*pNames) / 2 - 1;
       return nullptr;
     }
   }
@@ -210,7 +209,7 @@
       if (ppFind)
         *ppFind = pNames;
       if (pFindIndex)
-        *pFindIndex = i;
+        *pFindIndex = pdfium::base::checked_cast<int32_t>(i);
       if (iCompare < 0)
         continue;
 
@@ -257,7 +256,7 @@
   // The leaf node that holds `key` and `value`.
   CPDF_Array* container;
   // The index for `key` in `container`. Must be even.
-  int index;
+  size_t index;
 };
 
 // Find the `nTargetPairIndex` node in the tree with root `pNode`. `nLevel`
@@ -279,7 +278,7 @@
       return absl::nullopt;
     }
 
-    int index = 2 * (nTargetPairIndex - *nCurPairIndex);
+    size_t index = 2 * (nTargetPairIndex - *nCurPairIndex);
     CPDF_Object* value = pNames->GetDirectObjectAt(index + 1);
     if (!value)
       return absl::nullopt;
diff --git a/core/fpdfdoc/cpdf_structtree.cpp b/core/fpdfdoc/cpdf_structtree.cpp
index 26c3395..6519e1c 100644
--- a/core/fpdfdoc/cpdf_structtree.cpp
+++ b/core/fpdfdoc/cpdf_structtree.cpp
@@ -55,7 +55,7 @@
   if (pKids->IsDictionary())
     dwKids = 1;
   else if (const CPDF_Array* pArray = pKids->AsArray())
-    dwKids = pArray->size();
+    dwKids = fxcrt::CollectionSize<uint32_t>(*pArray);
   else
     return;
 
diff --git a/core/fpdfdoc/cpvt_section.cpp b/core/fpdfdoc/cpvt_section.cpp
index 4975316..a09a5e9 100644
--- a/core/fpdfdoc/cpvt_section.cpp
+++ b/core/fpdfdoc/cpvt_section.cpp
@@ -232,7 +232,8 @@
 
 CPVT_WordPlace CPVT_Section::AddLine(const CPVT_LineInfo& lineinfo) {
   m_LineArray.push_back(std::make_unique<Line>(lineinfo));
-  return CPVT_WordPlace(m_SecPlace.nSecIndex, m_LineArray.size() - 1, -1);
+  return CPVT_WordPlace(m_SecPlace.nSecIndex,
+                        fxcrt::CollectionSize<int32_t>(m_LineArray) - 1, -1);
 }
 
 CPVT_FloatRect CPVT_Section::Rearrange() {
diff --git a/core/fpdfdoc/cpvt_variabletext.cpp b/core/fpdfdoc/cpvt_variabletext.cpp
index 9dff62a..c14dda1 100644
--- a/core/fpdfdoc/cpvt_variabletext.cpp
+++ b/core/fpdfdoc/cpvt_variabletext.cpp
@@ -15,6 +15,7 @@
 #include "core/fpdfdoc/cpvt_wordinfo.h"
 #include "core/fpdfdoc/ipvt_fontmap.h"
 #include "core/fxcrt/fx_codepage.h"
+#include "core/fxcrt/fx_safe_types.h"
 #include "core/fxcrt/stl_util.h"
 #include "third_party/base/check.h"
 #include "third_party/base/compiler_specific.h"
@@ -266,11 +267,11 @@
   if (!m_SectionArray.empty())
     m_SectionArray.front()->SetRect(CPVT_FloatRect());
 
-  int32_t nCharCount = 0;
-  for (int32_t i = 0, sz = swText.GetLength(); i < sz; i++) {
-    if (m_nLimitChar > 0 && nCharCount >= m_nLimitChar)
+  FX_SAFE_INT32 nCharCount = 0;
+  for (size_t i = 0, sz = swText.GetLength(); i < sz; i++) {
+    if (m_nLimitChar > 0 && nCharCount.ValueOrDie() >= m_nLimitChar)
       break;
-    if (m_nCharArray > 0 && nCharCount >= m_nCharArray)
+    if (m_nCharArray > 0 && nCharCount.ValueOrDie() >= m_nCharArray)
       break;
 
     uint16_t word = swText[i];