iMac Not Printing PDF Files to HP LaserJet Printer

To restore PDF printing from your iMac, reset the macOS printing system to clear corrupted printer cache or driver communication errors. Word and Excel process simple text commands differently than complex graphic-heavy PDF structures, which often hang or fail if the queue or system-level communication protocol is corrupted. 

printing pdf files from what app?

many apps can open and print pdf files, preview, some adobe app(s) word can open the type of files too

Which HP printer model?

Please be specific about the name too, as HP has models with similar names for very different printers.

Which iMac and what macOS version is it running? Those often has a bearing on printing issues.

Also, older LaserJets may not have driver support for newer macOS versions, so please post the printer's model number as well.