Hope it helps

The pdf_viewer widget itself is usually not the root cause. The key clue is the error:

Unexpected server response (500) while retrieving PDF
/web/content?model=skills.matrix&field=certificate&id=18

A 500 error means Odoo failed while serving the binary content through the /web/content route.

Things to check:

1. Field definition

   Make sure the field is defined as a Binary field:

   certificate = fields.Binary(
       string="Certificate",
       attachment=True,
   )
   

2. Check Odoo server logs

   When opening the record, inspect the full traceback in the Odoo log. The actual exception causing the 500 error will be there.

3. Verify attachment access

   If attachment=True, Odoo stores the file in ir.attachment. Check whether the current user has access to:

   • The parent record (skills.matrix)

   • The related ir.attachment

   Access rule issues often cause files to load during upload but fail when reloading the record.

4. Test without attachment storage

   Temporarily change:

   certificate = fields.Binary(
       string="Certificate",
       attachment=False,
   )
   

   If the PDF loads correctly afterward, the issue is likely related to ir.attachment access or storage.

5. Verify the attachment record

   In Developer Mode, locate the corresponding ir.attachment and confirm:

   • res_model = skills.matrix

   • res_id = 18

   • The attachment file exists

   • The mimetype is application/pdf

6. Custom record rules

   If there are custom security rules on either skills.matrix or ir.attachment, they may prevent /web/content from reading the file after the page reloads.

Since the PDF displays immediately after upload but fails after revisiting the record, the file is most likely being saved correctly. The issue is usually related to attachment access rights, attachment metadata, or an exception raised by /web/content when retrieving the stored file.