Merge several PDF-Files with PDFtk

pdftk *.pdf  cat output bigfile.pdf

Crop PDF-Files with PDFCrop

pdfcrop --margins '30 20 30 20' --clip input.pdf output.pdf --verbose

Crop on Windows OS >> Use this ingenious tool: Briss

Convert JPG to PDF with ImageMagick

convert *.jpg output.pdf

With compression

convert -compress jpeg *.jpg output.pdf

In relation to the file name

for f in *.jpg; do convert ./"$f" ./"${f%.jpg}.pdf"

Convert PPT  to PDF with LibreOffice

libreoffice --headless --invisible --convert-to pdf *.ppt