fixes

app/core/src/main/java/stirling/software/SPDF/controller/api/MergeController.java

@@ -287,6 +287,7 @@ public class MergeController {
 
         boolean removeCertSign = Boolean.TRUE.equals(request.getRemoveCertSign());
         boolean generateToc = request.isGenerateToc();
+        boolean preserveAccessibility = request.isPreserveAccessibility();
 
         MultipartFile[] files = request.getFileInput();
         if (files == null) {
@@ -312,11 +313,12 @@ public class MergeController {
         try {
             PDFMergerUtility mergerUtility = new PDFMergerUtility();
             // OPTIMIZE_RESOURCES_MODE closes source documents progressively and skips
-            // structure-tree copying. Trade-off: PDF/UA tags (used by screen readers) are not
-            // preserved in the merged output. Most users don't have tagged PDFs and this trades
-            // negligibly-different output for measurably lower peak heap during merge.
+            // structure-tree copying — drops PDF/UA tags but uses much less heap.
+            // PDFBOX_LEGACY_MODE preserves tags at the cost of higher peak heap.
             mergerUtility.setDocumentMergeMode(
-                    PDFMergerUtility.DocumentMergeMode.OPTIMIZE_RESOURCES_MODE);
+                    preserveAccessibility
+                            ? PDFMergerUtility.DocumentMergeMode.PDFBOX_LEGACY_MODE
+                            : PDFMergerUtility.DocumentMergeMode.OPTIMIZE_RESOURCES_MODE);
             long totalSize = 0;
             File[] sourceFiles = new File[files.length];
             for (int index = 0; index < files.length; index++) {
@@ -340,6 +342,21 @@ public class MergeController {
             } catch (IOException e) {
                 ExceptionUtils.logException("PDF merge", e);
                 if (PdfErrorUtils.isCorruptedPdfError(e)) {
+                    // Identify which source file(s) are corrupt for operator diagnostics.
+                    // Only runs on the failure path so the happy path stays fast.
+                    List<String> badFiles = new ArrayList<>();
+                    for (int i = 0; i < sourceFiles.length; i++) {
+                        try (PDDocument ignored =
+                                pdfDocumentFactory.load(sourceFiles[i], true)) {
+                            // OK
+                        } catch (IOException corruptInput) {
+                            String name = files[i].getOriginalFilename();
+                            badFiles.add(name != null ? name : ("index " + i));
+                        }
+                    }
+                    if (!badFiles.isEmpty()) {
+                        log.warn("Corrupted PDFs in merge input: {}", badFiles);
+                    }
                     throw ExceptionUtils.createMultiplePdfCorruptedException(e);
                 }
                 throw e;
@@ -357,10 +374,6 @@ public class MergeController {
                 int[] pageCounts = generateToc ? collectPageCounts(sourceFiles) : null;
 
                 outputTempFile = new TempFile(tempFileManager, ".pdf");
-                // Hint the GC to reclaim merger transients before we open the merged document.
-                // The merger has just dropped its destination COSDocument; reclaiming that heap
-                // before loading the merged file again limits live-set during the modify pass.
-                System.gc();
                 try (PDDocument mergedDocument = pdfDocumentFactory.load(mergeOutput.getFile())) {
                     // Resource cache off for the modify pass — we never call getImage() here,
                     // and disabling it prevents PDFBox from caching XObjects when the page tree

app/core/src/main/java/stirling/software/SPDF/model/api/general/MergePdfsRequest.java

@@ -40,6 +40,16 @@ public class MergePdfsRequest extends MultiplePDFFiles {
             defaultValue = "false")
     private boolean generateToc = false;
 
+    @Schema(
+            description =
+                    "Flag indicating whether to preserve PDF/UA accessibility tags (structure"
+                            + " tree) in the merged output. When false (default) the merger runs in"
+                            + " resource-optimised mode which drops tags but uses significantly less"
+                            + " heap. Set true when merging tagged PDFs intended for screen readers.",
+            requiredMode = Schema.RequiredMode.NOT_REQUIRED,
+            defaultValue = "false")
+    private boolean preserveAccessibility = false;
+
     @Schema(
             description =
                     "JSON array of client-provided IDs for each uploaded file (same order as fileInput)",

frontend/src/core/components/tools/merge/MergeSettings.test.tsx

@@ -19,6 +19,7 @@ describe("MergeSettings", () => {
   const defaultParameters: MergeParameters = {
     removeDigitalSignature: false,
     generateTableOfContents: false,
+    preserveAccessibility: false,
   };
 
   const mockOnParameterChange = vi.fn();
@@ -86,6 +87,13 @@ describe("MergeSettings", () => {
       "generateTableOfContents",
       true,
     );
+
+    // Click the third checkbox (preserveAccessibility - should toggle from false to true)
+    fireEvent.click(checkboxes[2]);
+    expect(mockOnParameterChange).toHaveBeenCalledWith(
+      "preserveAccessibility",
+      true,
+    );
   });
 
   test("should call translation function with correct keys", () => {
@@ -107,5 +115,9 @@ describe("MergeSettings", () => {
       "merge.generateTableOfContents.label",
       "Generate table of contents in the merged file?",
     );
+    expect(mockT).toHaveBeenCalledWith(
+      "merge.preserveAccessibility.label",
+      "Preserve accessibility tags? (uses more memory on large merges)",
+    );
   });
 });

frontend/src/core/components/tools/merge/MergeSettings.tsx

@@ -50,6 +50,21 @@ const MergeSettings: React.FC<MergeSettingsProps> = ({
         }
         disabled={disabled}
       />
+
+      <Checkbox
+        label={t(
+          "merge.preserveAccessibility.label",
+          "Preserve accessibility tags? (uses more memory on large merges)",
+        )}
+        checked={parameters.preserveAccessibility}
+        onChange={(event) =>
+          onParameterChange(
+            "preserveAccessibility",
+            event.currentTarget.checked,
+          )
+        }
+        disabled={disabled}
+      />
     </Stack>
   );
 };

frontend/src/core/hooks/tools/merge/useMergeOperation.ts

@@ -33,6 +33,10 @@ const buildFormData = (
     "generateToc",
     (parameters.generateTableOfContents ?? false).toString(),
   );
+  formData.append(
+    "preserveAccessibility",
+    (parameters.preserveAccessibility ?? false).toString(),
+  );
 
   return formData;
 };

frontend/src/core/hooks/tools/merge/useMergeParameters.ts

@@ -7,11 +7,13 @@ import {
 export interface MergeParameters extends BaseParameters {
   removeDigitalSignature: boolean;
   generateTableOfContents: boolean;
+  preserveAccessibility: boolean;
 }
 
 export const defaultParameters: MergeParameters = {
   removeDigitalSignature: false,
   generateTableOfContents: false,
+  preserveAccessibility: false,
 };
 
 export type MergeParametersHook = BaseParametersHook<MergeParameters>;