This is very easy with the PDFen tool:

1) Upload your Powerpoint files in the tool by clicking on 'Upload'.

2) If you want to convert it to a PDF/A then check this option, if you don't want this, leave this option unchecked.

3) Click "Convert" to convert all Powerpoint files to separate PDF files or click Merge to merge all Powerpoint files to 1 PDF.

4) Click on "download", you will receive a zip file with all separate PDF files or the merged PDF.

PDF docs
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