While the other answers address the legal issues better than I can, there’s some important context to this issue that is missing from the other answers which I’d like to discuss.
As everyone knows, the Impeachment process has 2 stages: first, the House writes and votes on Articles of Impeachment, then the Senate holds a trial on them. Since the Articles of Impeachment have been passed, the first stage is over, but before the trial can start, the Senate must first vote on the rules that will govern that trial. This is the stage we’re at now, and Speaker Pelosi’s gambit is part of the negotiations (which have been ongoing for some time) about what these rules will be.
The concern, of course, is that Senator McConnell will push through rules that would create a “kangaroo court” designed to assure Trump’s acquittal. For example, Schumer and McConnell have been clashing on whether to allow witnesses at the trial, with McConnell rejecting that and offering vague assurances that the issue could be revisited during the trial:
Senate Majority Leader Mitch McConnell on Tuesday rejected calls from Senate Minority Leader Chuck Schumer to allow witnesses at an expected Senate impeachment trial of President Donald Trump ...
McConnell's rejection of Schumer's request does not mean the question of witnesses is settled. McConnell said the Senate should revisit the issue at some point in the middle of the trial, similarly to how it was handled in the Clinton impeachment trial.
McConnell rejects Schumer's call for witnesses at impeachment trial
Holding up the start of the Senate trial grants more time for these negotiations to play out, which Congressional Democrats clearly believe would be helpful for their case:
Senate Democrats are waging a pressure campaign to try get their GOP colleagues to break ranks on impeachment witnesses.
Absent a deal with Senate Majority Leader Mitch McConnell (R-Ky.), Democrats are expected to force floor votes on their requests for trial documents and witnesses, including acting White House chief of staff Mick Mulvaney and former national security adviser John Bolton.
Senate Minority Leader Charles Schumer (D-N.Y.), backed by members of his caucus, is working to put Republicans in a bind and drive a wedge between GOP leaders, who say they don’t want witnesses, and a handful of moderate and retiring senators viewed by Democrats as persuadable on procedure.
Schumer aims to drive wedge between Republicans on impeachment
So, while I think the answer by Just Me answers the legal question better than I can, I’d argue that it’s better to think of this as a political gambit that is almost certainly going to have a political resolution, rather than a legal one
[Speaker Pelosi] did not indicate that she was contemplating holding the articles forever.
Ms. Pelosi and Mr. Schumer were planning to meet later on Thursday morning to discuss their next steps.
While she did not say explicitly what she believes would constitute a fair trial, she indicated she would support the plan laid out by Mr. Schumer.
“We’d like to see a trial where it’s up to the senators to make their own decisions and working together, hopefully, in recognition of witnesses that the president withheld from us, the documents that president withheld from us,” Ms. Pelosi said.
In an interview on Thursday, Pelosi expanded on her thinking. She explicitly shot down the idea of an indefinite delay, and stated that she is waiting to declare impeachment managers until the Senate determines what the rules of the trial will be:
Pelosi, in a wide-ranging interview, said she can’t name impeachment managers — the House Democrats who will essentially serve as prosecutors in the Senate trial — until she knows the terms of the proceedings.
"We said what we’re going to say. When we see what they’ll do, we’ll know who and how [to appoint]," Pelosi said Thursday afternoon.
The terms will ultimately be set by Senate Majority Leader Mitch McConnell (R-Ky.) and Minority Leader Chuck Schumer (D-N.Y.). The two will need to reach an agreement on the parameters of the trial — including the time frame, witness testimony and obtaining documents — that will require 51 votes. Democrats have questioned the impartiality of the Senate trial after McConnell said he’s coordinating with the White House on strategy.
Pelosi blamed McConnell for the impasse, saying he was the one who wasn't abiding by precedent set during President Bill Clinton's impeachment trial in 1998.
"They had a bipartisan bill with 100 senators voting for it, that’s the precedent. That’s not what McConnell was saying," Pelosi told POLITICO. "But let’s give them the chance to do what they have to do over there. And so until they do, there’s really not that much to talk about."
In her weekly press conference earlier Thursday, Pelosi essentially dismissed the idea that she would hold onto the articles of impeachment indefinitely.
“We’re ready,” Pelosi told reporters Thursday about moving forward after the House impeached Trump, charging him with abuse of power and obstruction of Congress. “When we see the process that is set forth in the Senate, then we’ll know the number of managers that we may have to go forward and who we will choose."
Pelosi downplays threat to withhold impeachment articles from Senate