parts of my technical drawings are missing when converting from keynote to pdf

Although you can insert a PDF into Keynote, it will only display the first page of that PDF should it be a multi-page PDF. Also, if the PDF contains some transparency, or PDF functionality not supported by Apple's PDFContext translation during the export process , that may result in omission of drawing content.

Consider adding the technical PDF drawings with a proper PDF editor after the Keynote export to PDF. That should avoid any drawing omissions. I suggest this as you will not have any control over the Keynote to PDF translation process by any other means.

The PDF handling in Apple applications, or its Preview application is not as comprehensive as it would be in Adobe Acrobat Pro or even Microsoft applications. Even if single-page PDFs, if you have changed the tool that you use to create those technical drawings, or the vendor has changed its PDF generation functionality in a newer product version, you may run into issues when exporting those embedded PDFs from Keynote to PDF.

You have zero control over the black-box PDF export engine in Apple's applications and if there is an inherent compatibility issue, you will then need to follow my previous advice about a PDF editor as a post-processor for those technical drawings.