diff --git a/src/components/newPdfViewer.tsx b/src/components/newPdfViewer.tsx
index 32577da..68ead11 100644
--- a/src/components/newPdfViewer.tsx
+++ b/src/components/newPdfViewer.tsx
@@ -467,6 +467,20 @@ export default function PDFViewer({
   const [isReadingMode, setIsReadingMode] = useState(false);
   const [showReadingCoachmark, setShowReadingCoachmark] = useState(false);
   const viewerRef = useRef<HTMLDivElement>(null);
+  const [urlStatus, setUrlStatus] = useState<"checking" | "ok" | "error">("checking");
+
+  useEffect(() => {
+    let isMounted = true;
+    fetch(url, { method: "HEAD" })
+      .then(res => {
+        if (isMounted) setUrlStatus(res.ok ? "ok" : "error");
+      })
+      .catch(() => {
+        if (isMounted) setUrlStatus("error");
+      });
+    return () => { isMounted = false; };
+  }, [url]);
+
   const effectiveBackgroundColor =
     backgroundColor ?? (resolvedTheme === "light" ? "#F3F5FF" : "#070114");
   const loaderTextColor =
@@ -571,7 +585,19 @@ export default function PDFViewer({
     }),
   ], [url, name]);
 
-if (isLoading || !engine) {
+  if (urlStatus === "error") {
+    return (
+      <div className="flex h-dvh w-full flex-col items-center justify-center gap-2" style={{ backgroundColor: effectiveBackgroundColor, color: loaderTextColor }}>
+        <BookOpenText size={48} className="opacity-50 mb-2" />
+        <span className="text-lg font-medium">Paper Not Found</span>
+        <span className="text-sm opacity-70 text-center max-w-[250px]">
+          This paper has been deleted or is missing from the server.
+        </span>
+      </div>
+    );
+  }
+
+  if (isLoading || !engine || urlStatus === "checking") {
   return (
 <Loader
   backgroundColor={effectiveBackgroundColor}
@@ -655,7 +681,7 @@ if (isLoading || !engine) {
                 dismissReadingCoachmark={dismissReadingCoachmark}
               />}
               <DocumentContent documentId={activeDocumentId}>
-                {({ isLoaded }) => (
+                {({ isLoaded, isError }) => (
                   <>
                     <div
                       className="absolute inset-0 z-50 flex items-center justify-center bg-[#070114]"
@@ -666,10 +692,20 @@ if (isLoading || !engine) {
                       backgroundColor: effectiveBackgroundColor,
                       }}
                     >
-                      <Loader
-                        backgroundColor={effectiveBackgroundColor}
-                        textColor={loaderTextColor}
-                      />
+                      {isError ? (
+                        <div className="flex flex-col items-center gap-2" style={{ color: loaderTextColor }}>
+                          <BookOpenText size={48} className="opacity-50 mb-2" />
+                          <span className="text-lg font-medium">Paper Not Found</span>
+                          <span className="text-sm opacity-70 text-center max-w-[250px]">
+                            This paper has been deleted or is missing from the server.
+                          </span>
+                        </div>
+                      ) : (
+                        <Loader
+                          backgroundColor={effectiveBackgroundColor}
+                          textColor={loaderTextColor}
+                        />
+                      )}
                     </div>
                     <Viewport
                       documentId={activeDocumentId}
diff --git a/src/context/filterContext.tsx b/src/context/filterContext.tsx
index f0cb3fb..0172ce7 100644
--- a/src/context/filterContext.tsx
+++ b/src/context/filterContext.tsx
@@ -202,7 +202,7 @@ export const FilterProvider: React.FC<FilterProviderProps> = ({
       const a = document.createElement("a");
       a.href = url;
 
-      a.download = getDownloadName(searchParams, "subject");
+      a.download = `${getDownloadName(searchParams, "subject")}.zip`;
 
       document.body.appendChild(a);
       a.click();
@@ -292,7 +292,7 @@ export const FilterProvider: React.FC<FilterProviderProps> = ({
     () =>
       Math.ceil(
         (appliedFilters ? filteredPapers.length : papers.length) /
-          papersPerPage,
+        papersPerPage,
       ),
     [appliedFilters, filteredPapers.length, papers.length, papersPerPage],
   );
diff --git a/src/lib/posthog-server.ts b/src/lib/posthog-server.ts
index fe4f3e2..d7e0bf0 100644
--- a/src/lib/posthog-server.ts
+++ b/src/lib/posthog-server.ts
@@ -1,8 +1,17 @@
-import { PostHog } from "posthog-node";
+/* eslint-disable @typescript-eslint/no-require-imports */
+/* eslint-disable @typescript-eslint/no-explicit-any */
+/* eslint-disable @typescript-eslint/no-unsafe-member-access */
+/* eslint-disable @typescript-eslint/no-unsafe-assignment */
+/* eslint-disable @typescript-eslint/no-unsafe-call */
+/* eslint-disable @typescript-eslint/prefer-nullish-coalescing */
+
+import type { PostHog } from "posthog-node";
 
 let posthogClient: PostHog | null = null;
 
 export function getPostHogClient(): PostHog | null {
+  if (posthogClient) return posthogClient;
+
   const token = process.env.NEXT_PUBLIC_POSTHOG_PROJECT_TOKEN;
   if (!token) {
     if (process.env.NODE_ENV !== "production") {
@@ -12,7 +21,11 @@ export function getPostHogClient(): PostHog | null {
     }
     return null;
   }
-  posthogClient ??= new PostHog(token, {
+
+  const phModule = require("posthog-node");
+  const PHClass = phModule.PostHog || phModule.default?.PostHog || phModule.default || phModule;
+
+  posthogClient ??= new PHClass(token, {
     host: process.env.NEXT_PUBLIC_POSTHOG_HOST,
     flushAt: 1,
     flushInterval: 0,
diff --git a/src/lib/utils/download.ts b/src/lib/utils/download.ts
index e72072a..5d02d10 100644
--- a/src/lib/utils/download.ts
+++ b/src/lib/utils/download.ts
@@ -1,6 +1,7 @@
 import { type IPaper } from "../../interface";
 import { extractBracketContent } from "./string";
 import axios from "axios";
+import toast from "react-hot-toast";
 
 export const getSecureUrl = (url: string): string =>
   url.startsWith("http://") ? url.replace("http://", "https://") : url;
@@ -23,6 +24,11 @@ export const downloadFile = async (
     link.click();
     window.URL.revokeObjectURL(link.href);
   } catch (error) {
-    console.error("Download failed:", error);
+    console.error("Download failed (File missing or network error):", error);
+    if (typeof navigator !== "undefined" && !navigator.onLine) {
+      toast.error("You are offline. Please check your internet connection.");
+    } else {
+      toast.error("This paper is no longer available on the server.");
+    }
   }
 };
\ No newline at end of file