PDF to GIF compares the uploaded file's extension against .pdf and returns an "Invalid file type" error before any conversion when it doesn't match.
OWASP's own File Upload Cheat Sheet recommends checking an uploaded file's extension against a whitelist of allowed extensions rather than trying to block a list of forbidden ones. File Upload - OWASP Cheat Sheet Series
PDF to GIFPDF to GIF Rejects File Invalid File Type Error
The check reads the file's original uploaded name when available, falling back to the server-generated name only if it is missing.
Rename the file so its name ends in .pdf.
Upload the renamed file to the PDF to GIF form.
Renaming Files for PDF to GIF Uploads
Open the folder containing the rejected file.
Select that same file and then choose the rename option from the menu.
Type the new name ending in .pdf and confirm the change.
Continue with Select Specific Pages or Single Frame Output.
Frequently Asked Questions about PDF to GIF Invalid File Type Error
How do I fix the invalid file type error?
Rename the file to end in .pdf and upload it again.
Where do I upload the corrected file?
Upload the file using the PDF to GIF form.
How do I verify the file extension?
Check the file name in your file manager to confirm it ends in .pdf.
What if the file is actually a PDF but has a different extension?
Rename the file to end in .pdf before uploading.
Does the error mean my file is corrupted?
Treat the error as a naming issue first, since a mismatched extension alone triggers it.