Document feeder takes more than on sheet at a time.

Yep, only when you use the document feeder? If so, lift the document feeder lid, get a clean lint-free cloth and some glass cleaner. Apply some of the cleaner to the cloth then, clean the small of the two glass areas. That is the only section that reads documents fed through the document feeder! Run a blank sheet of paper through the document feeder first before cleaning and make a copy. Take the copy and, if there is a line (or lines)on the copy, place it on the larger document glass face up.Follow the line to the smaller glass. The dot of debris will usually be there. If you don't see anything wrong, clean the glass anyway. Run another blank sheet through the document feeder. Check that copy. If the lines are gone...Viola! you have solved the problem. It it has not and you've cleaned it more than once, the debris is on the mirrors under the glass and a technician will need to come out, remove the glass and carefully clean the optics/mirrors.
Good Luck.

Load an original in the automatic document feeder (ADF)
You can copy, scan, or fax a single or multiple-page A4, letter, or legal-size document by
placing it in the document feeder tray.
The document feeder tray holds a maximum of 20 sheets of letter or A4-size paper or 15
sheets of legal-size paper.
To send a basic fax from the control panel
1. Load your originals. For additional information, see Load originals.
2. Enter the fax number by using the keypad, press Speed Dial or a one-touch speed
dial button to select a speed dial, or press Redial/Pause to redial the last number
dialed.
TIP: To add a pause in the fax number you are entering, press Redial/Pause,
or press the Symbols (*) button repeatedly, until a dash (-) appears on the display.
3. Press START FAX Black.
If the device detects an original loaded in the automatic document feeder, it
sends the document to the number you entered.
TIP: If the recipient reports issues with the quality of the fax you sent, you can
try changing the resolution or contrast of your fax.

for further refernce check out the online manual
http://h10032.www1.hp.com/ctg/Manual/c01181836.pdf

The most common cause of blank (void) lines on a scanned document is from whiteout on the glass where the page is scanned when using the document feeder.
If there is a clear mylar sheet on the bottom of the document feeder, the whiteout may be there instead.

If there was dirt or a dark smudge on the glass (or mylar), it would show up as a black line, but whiteout is white, and shows up as a void (blank) line.

if using the feeder probably face up as it will likely pull the document in and invert it to lay face down on the scanning glass.

if you lift up the feeder (like lifting the top of a copy machine), then it would be face down on the glass.

there should be an image on the feeder illustrating how the document should be placed - if it looks like a page with lines on it with a bent corner then it is face up.

Take a little soap and a little water and clean the gray feed tire that is located in the document feeder. Check for any paper obstructions. If the machine has had a lot of prints I would replace the feed tire.

http://www.websitetoolbox.com/mb/copierquestions

When you load the document feeder, simply place all of the sheets, up to I believe 25 sheets at a time in the document feeder. The doc feeder, if working correctly will load them one at a time and then exit them to the receiving shelf or tray on your printer.

If you do not have a document feeder, or must use the manual scan function for some reason, there is also a FAX multiple pages function from the sub menu.

ZBob

hello!

have you tried cleaning the separator pad inside the automatic document feeder?

try to Load one or two sheets of unused plain white full-size paper into the document feeder tray, and then press Start Copy Black to pull the pages through the automatic document feeder. The plain paper releases and absorbs residue from the rollers and separator pad.

If the problem persists, clean the rollers and separator pad manually

There is a thin strip of glass that you will see when you lift the top feeder. There is most likely white-out on it.

When you use the top feeder, it doesn't use the large glass, it only 'rolls' the original document over this small strip of glass.

clean the big rubber roller on the document feeder with windex applied to a micrfiber cloth and then try to fax or copy two sheets

Its most likely to be the pickup roller and separation pad which will need replacing. as a short term fix try cleaning them with some isopropyl alcohol